I've been experiencing the same thing for awhile. The illustration below displays the ion-icons in .svg format, as an example. The folder on the left is syncing with the Synology drive, the one on the right is non-syncing common downloads folder.
Apparently something is broken in the first case. So - whenever I'm working on something, I need to make a copy to some non-syncable folder.
Anything that pops to your mind that I may try to fix this issue?, apart from stop syncing...
Thanks in advance 
Does this also happen in File Explorer?
Opus has no SVG code inside it at all, so whatever is generating the SVG thumbnails is not part of Opus. I don't know why it would have problems with a Synology drive, but there are sometimes strange differences between Windows and third-party NAS devices/servers.
Interesting. In File Explorer they are fine, but I need to refresh it for couple of times since there are around 700 icons in there. After that, they start to appear in Opus, as well, just to be broken again...
I just took a look, I used this extension: https://github.com/tibold/svg-explorer-extension/releases
Also, I just noticed there is new version available. Don't believe it would help, but let's try...
Anyway, some sort of SVG support should be available, as today's web development is practically unimaginable without it. I'm glad the WebP is working out of the box, though.
UPDATE: I just tried with and without the extension. Also tried to uninstall IrfanView. Put the newest available. In the end Opus shows the same behaviour. File Explorer works fine.
It works even without the extension? It's probably not the extension generating the thumbs in that case. Or you're looking at cached thumbs, which can confuse matters a lot.
The cloud storage status icons on the files also suggest that the thumbnails may be coming from the drive and not the local machine, since that's how OneDrive works and they may be using the same mechanism.
Have you checked what happens in another program that isn't below the Windows directory? e.g. Try the File > Open dialog in a third party text editor (not Notepad). Those (like Opus) can be treated differently by the cloud storage API, although we have a possible solution on the way if it is that.
No it is not. I was only testing if it would since I updated Windows to 2004 build.
If i understand correctly, in this case I'd be having problems in File Explorer to, which is not the case. It works properly there.
Microsoft broke parts of the newer cloud storage APIs (not everything uses them, of course) by default in programs outside of the Windows directory. File Explorer isn't affected. (It's something we're working on.)
I have just now. You're right. Not really working. Apparently the icons are somehow cached, and the system pulls out from there. Something is very fishy going on here...
@leo I tried to delete the icon cache from %AppData%\Local, however Opus is still pulling the same icons from some cache. How do I delete Opus icon cache if such thing exists at all?
Deleting the icon cache manually may not work unless the things using it are shutdown at the same time. You'd also probably need to delete the thumbnail cache not the icon one.
Try this, which should delete both the system icon and thumbnail caches, and restarts Explorer and Opus for you: Clear Icon Cache
There's also an Opus-specific thumbnail cache which you can clear under Preferences / File Display Modes / Thumbnails, but it would not normally be used for thumbnails which came via the shell.
Maroon screen of death lol, need to restart the system. 
Right - makes no difference really.
Meh - looks like I'll need to live with what I have for the time being. Shame, SVG icons are really a way to go these days.
Thanks for help @leo.
Let me just report back that the things got sorted by themselves in the meantime. I'm pretty sure it has nothing to do with the SVG extension as I haven't updated it since. So - it's either some Windows update or some of the latest Opus releases (throw some tomatoes on me, I haven't read the update logs).
Anyway, works perfectly now with Synology synced folders. Examples below.
