Thanks for the detailed report, I will investigate !

Categories are just a way of organising your library.

There is no difference between categories regarding file format, or anything else.

That's part of why the development took so much time. On previous versions, books and comics were managed completely separately, with different code. Now every file is managed exactly the same way (with specific behaviour based on specific formats, e.g. the online reader, not categories). 

As for the "Calibre library" checkbox: when creating its internal folder structure, Calibre creates a folder for each book, which is annoying when you display the folder structure in Ubooquity.

Checking the box just hides the intermediate folder, removing an unnecessary click when you navigate.

You are using Java 8, the new version of Ubooquity requires at least Java 17, hence the crash.

Thanks for the feedback everyone ! :)

Could you give me the exact version of Java you use ?

Also, the few log lines after the "A java Exception has occurred" would help too.

As for rar5, support depends on the RAR library used by Ubooquity (Junrar). I'm not sure it will be ever implemented: https://github.com/junrar/junrar/issues/23

Given that RAR files have no advantage over ZIP files for comics, I don't understand why this format is still in use though.

Indeed, thanks !

Should be fixed in the official beta:

https://vaemendis.net/ubooquity/article23/ubooquity-3-beta

I tested the file you sent me: the file is correctly imported and can be read online.

I unfortunately don't know what is the cause of the issue you encounter.

Do you have any error in Ubooquity logs when the "page not found" is displayed ?

If you can send me an example of DJVU file that can't be opened, I'll take a look to try to understand what's wrong. My address: tom 'at' vaemendis.net

The online reader is already usable with DJVU files. 
What issue did you encounter ?

Altso, in current version of Ubooquity (2.x), the Djvu files have to be in the "Books" category.