Were there some specific error in the log file (look for the name of the files which failed) ?

If you want, send me the files that don't work and I'll take a look.
tom "at" vaemendis.net

OPDS is not supposed to be used in browsers.
Firefox offers a basic representation of the OPDS XML file, but it's a lot let usable than the usual Ubooquity pages.

OPDS is a way to allow inter applications communication. Especially for mobile applications (cf Ubooquity 1.7.0 annoucement post on the site).

Here are a few clarifications on the matter.

All images Ubooquity provides, be they thumbnails or full pages, are jpeg generated by Ubooquity itself. So there won't be any browser compatibility problem (at not point does Ubooquity just "forward" the images, they are all read and resized).

Webp support requires a decoder. "webp-imageio" is not a candidate as it is only a wrapper around native code. Using it would require different version of Ubooquity depending on the platform you run it on (Linux, Windows, Mac OS...). Not an option.

I have howver found a free, full java Webp decoder. I will test it and add Webp support if it works well enough. That is if I find the time to do it. As you might have noticed, Ubooquity development has been even slower than before and I won't have much time to give it to in the coming months.

Sorry I could'nt help more. I still don't have any idea of what happened.

This one is a weird bug. I have no idea of what might have led to your situation. 

Each comic book is identified by an ID number in the database. This ID is also used to name the thumbnails files. Looks like the IDs in the database and the ones in the thumbnails cache folder are no longer in sync.
I'm afraid your only solution is a full rescan of your collection. :/

I'll try to reproduce the problem on my side, thanks.