What do you mean by turning on/off Adobe Reader in plugins? Do you mean within the Opus ActiveX viewer plugin? That would not affect thumbnails.
Do you see thumbs for PDFs on OneDrive in Explorer? If not, Opus isn't a factor. If you see them in Explorer but not Opus, have you installed Opus 12.6.3 and the Windows 10 Fall Creators Update?
Well, the one constant in your tests is OneDrive, so to me it's pretty clear that that's the issue. For what it's worth, I have checked on my machine, and thumbnails display fine in OneDrive folders. However, the one difference that may be relevant here is that I do not have the option to keep files offline enabled. If you can do that, it may be worth a try just disabling that option in OneDrive, and see if that changes anything.
Otherwise I'm out of ideas, unfortunately. Other than more radical stuff, like trying a fresh install of Windows 10 (and fresh install of DOpus), perhaps in a virtual machine, and see if and when the problem reappears. Yeah, major hassle I know, but it could help you pinpoint the cause of your issue.
I take your point about the Opus Active X viewer, but I wanted absolutely nothing to interfere with thumbnails. As nobody seems to know what produces thumbnails at any given point.
I do see the thumbs in Explorer and I have installed Opus 12.6.3 and The Fall Creator's update.
Pirx, I will give your suggestion a try.
It's different depending on what you have installed and what's in your registry, but hard to know from here.
The ActiveX Viewer Plugin config won't affect it so don't worry about that variable. (Some viewer plugins do affect thumbnails, but not that one.)
At last some progress.
I completely uninstalled Opus and removed its registry entries.
I reinstalled and looked at One Drive in the "virgin" installation. All the thumbs were there instantly.
I restored my working config nd the problem returned. I went back to the "virgin" install and they were back.
Any ideas how I might try to debug what is wrong with my installation. I would hate to have to rebuild it. Is there a way of taking bits out and then testing and pitting them back?
Install anything PDF related one at a time until it stops working.
Jon, it sounds like something in his Opus configuration is causing the problem. I understood him saying that with a fresh install of Opus, thumbnails are fine, but once he restored his working Opus configuration they are gone. So it seems to be an Opus issue after all.
Was using Settings > Backup & Restore literally all you did at those stages, or are there some extra things that were restored (e.g. 3rd party plugins)?
Try comparing the File Display Modes > Thumbnails settings between the two configs, and which viewer plugins are enabled. That's where I would start.
I back up and restore via the Opus panel in the conventional way.
I already tried making the Thumbnail settings the same between the two configs but to no avail.
The problem I am convinced is a registry problem brought on by Mystic Thumbs. If I uninstall Opus and Mystic thumbs and then reinstall Opus, I can see One Drive thumbnails fine.
The problem I have then is that whatever is producing the PDF thumbnails now Mystic Thumbs has gone does a dreadful job, thumbnailing about one in five pdfs and at a snail's pace.
When I reintroduce Mystic Thumbs, at first all seems fine I see the previews. But if I leave things for a short while and refresh the One Drive PDF folder, the thumbnails are gone.
Mystic Thumbs is increasing looking like the fly in this ointment. I will need to contact them.
When I get a reply or some progress I will report back, but judging from previous experience, it would be unwise for anybody to hold their breath.
So the Opus config wasn't involved in the end? Restoring or resetting the Opus config would not affect the registry.
There seems to be some kind of conflict between Mystic Thumbs and Opus in the area of One Drive PDF thumb creation. I can only surmise this ( I do not really know) because when I remove Mystic Thumbs from my machine (registry entries et al) the situation is not improved until I do a fresh install of Opus. I am assuming this is because Opus is putting something back into the registry that Mystic Thumbs screwed up. But I really do not know.
I always thought image metadata was a dog's breakfast, but thumbnail creation is even worse.
Thanks to all who offered their help. I await a response from the Mystic Thumbs developers in Brisbane.
Opus doesn't touch anything to do with PDF types/thumbnails in the registry. I doubt it is that. Probably something else that's being done at the same time, like restarting Opus and/or the system, or clearing the Opus or shell thumbnail caches.
I am happy to say that I am well on he way to fixing this problem - though I still have no idea really what the problem is. I suspect that somewhere is my config there is some corruption. I am currently painstakingly re-creating my config. I am also making backups at every stage in he hope I can catch the real problem.
Once again, many thanks to all who offered their thoughts and advice.
I started this as a new topic from my original posting of OneDrive PDF thumbnails because most everything in that topic turns out to be a load of old tosh.
After rebuilding my configuration, uninstalling all progs relating to pdfs and thumbnails, I finally got OneDrive PDFs to show up as thumbnails and create transparent PDF thumbnails at the same time.
My joy was unbounded until I tried to set the size of the thumbnails from 256 to 320. Low and behold they disappeared,
That is the real problem om my system. Set OneDrive PDF thumbnails to anything above 256 and they simply vanish to be replaced to icons.
Any size up to 256 is just fine.
I've merged the topics, so that people who find the larger topic don't miss the conclusion with the useful information in it.
Sounds like Adobe Reader may fail if asked to generate a thumbnail larger than 256x256, and whoever Adobe outsource their shell extension code to may not have heard of high DPI yet. 
It took them about 10 years to work out 64 bit so I am not surprised.
I have now categorically proved that this is the problem. I have load my original "problem" configuration where thumbnail size is set to 320.
The second I reduce this size to 256 the thumbnails appear.
Is it possible to make a lister that sets thumbnail size. Being limited to 256 is frustrating. If I could set the thumbnail size on individual folders I could have the best of both worlds.
Is this affecting PDFs in all folders or just the OneDrive folder?
If the Adobe thumbnailer fails when the size is too large then I would expect it to be the same in all folders.
Test with new, never before thumbnailed, copies of existing files to make sure you aren't getting cached thumbnails.
The problem only concerns One drive folders with PDFs in them. I can set the thumbnail size to anything up to the maximum I have nominated in Opus (512) on any other folder - including other folders containing PDFs and there is no problem. When I return to the One Drive PDF folders there are no thumbnails until I reduce the size of them to 256.
(This all makes perfect sense, given that they always work in Explorer as it has extremely limited functionality to control the size of the thumbnails)
This happens when I put new PDFs on ONe Drive. I have cleared the windows and Opus thumbnail cache to test and the situation is still the same.
I did get a system message saying that the default program for PDFs had been changed by Windows 10 to Microsoft Edge because the previous default program (Acrobat Reader DC was causing a problem). I have switched it back to Acrobat Reader with no discernible effects. The problem is still there.
I produce thumbs using Mystic Thumbs, but I strongly suspect that program is not producing the thumbs for One Drive, as they do not appear in the list of thumbs the program says it has made.
The strange thing is that as part of debugging this problem I removed acrobat from my machine and my PDF editor (Power PDF by Nuance) also. I still got the OneDrive thumbs.
When I put them back I did notice that my previously transparent PDF thumbs had disappeared, because Nuance was now producing them, I suppose. I had to do a re-install without any "integration" to cure that problem.
Also, you talk about Adobe thumbnailer, but way back in this thread Jon said he did not have Adobe reader on his machine. Is the Adobe thumbnailer a separate program from reader?
This whole thumbnailing business is such an integral part of modern windows system, it is a shame it has to be so confusing.
I think the latest versions of Windows 10 will produce PDF thumbnails without any third-party software installed (somehow through capabilities that Edge already has).
I think that makes sense, Pirx. I have been watching pdf thumbnails build on my system. If I build them in a directory on one of my drives I can see Mystic Thumbs reporting that it is building the thumbnails. When I push PDF files onto my One Drive folder I get no such report, indicating that something else is producing the thumbs.
The real question is what is stopping Opus from increasing the viewing size of these particular thumbnails?
Still, at least we know there is a problem. Perhaps people cleverer than me can solve it.