And thumbnail is generated in DOpus but not in Explorer. Picture in question is in JPEG XL format and it seem that it is handled by JPEG XL Viewer Plugin 0.0.0.4 (picture is not shown in Viewer if the plugin is disabled in Preferences).
Directory Opus Pro 12.33 Build 8659 x64
OS 6.1 (B:7601 P:2 T:1) SP 1.0 "Service Pack 1"
Lighthouse.7z (98.4 KB)
JPEG XL Viewer Plugin isn't made by or installed by us. Please talk to the plugin author about it.
Just a warning, Windows Defender flagged Lighthouse.7z as malware. What is inside it?
File saved by IrfanView as Lighthouse.jxl .
The zip only contains one small .JXL file and a reminder that Defender is worthless trash made by a lost company. 
Looks like a false positive then.
For what it's worth, the file displays perfectly for me in DOpus, in the viewer and as a thumbnail.
For me too, but not in Viewer Pane.
What does it show instead? Usually when this happens to me, the preview is being "hijacked" by another plugin which then fails to render it, leaving an empty panel. Sometimes right clicking will show you which plugin DOpus attempted to use.
Does it help by raising the priority of the JPEG-XL plugin until it is first in your plugin list?
I have disabled all plugins but JPEG XL and restarted DOpus. Viewer Pane shows blank page. Nothing after right clicking.
Plugins get called with slightly different flags when it's the viewer pane instead of the standalone viewer. It may come down to the plugin being called in a way it doesn't expect, perhaps.
Inside and outside of archives is another place where there could be a difference, if the plugin asks not to be called for slow or non-seeking streams.
Clutching at straws, but I assume you've tested it with a range of different JPEG-XL?
https://people.csail.mit.edu/ericchan/hdr/jxl_images/
I wouldn't just rely on files converted by Irfanview.
Strange thing. Once I downloaded one file from site given by xraale, then using "Show in folder" in browser, all .xls (including Lighthouse.xls) are displayed in Viewer Pane. But if try to display them from Libraries/Pictures (Libraries/Obrazy in my installation), situation is as described earlier.
Seems like it makes a difference whether they are in lister lib://Pictures (Viewer Pane does not show them) or in lister C:\Users\Username\Pictures (Viewer Pane shows them). Even if it is physically the same location.
I have seen program that was unable to write to Libraries\Documents, but that was able to write to Libraries\Documents\My Documents (that is physically the same location).