Your comments

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:

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

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'

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.

Yes, the folder pictures were found using a very inefficient way that required several database requests. Now everything is done in a single requests, which completely ignores the user settings regarding sorting. 🤦‍♂️

I'll try to fix that !

Here you go:

Java 17 (or more recent) is required.

The documentation on the new theme system:

And a description of the data (variables) made available to the theme, by page:

The variables are very unlikely to change before the release of the beta.
The default theme will (HTML and definitely CSS), as it is an awful mess in its current state.

Let me know if you have any question !

That's the point of the beta version. :)

Binary's ready, I just have to finish the doc which explains how the templating system works.

I'll post it here as soon as it's ready (and write a release post later).

The most visible change is the display of the reading progress on books (by books I mean comics as well). 

But to do it I completely rewrote the way themes are managed. Until now you could only change the style and icons of Ubooquity with a custom theme. With V3, the whole HTML structure can be customized. This means a lot more freedom in the way collections can be displayed.

I also merged the "books" and "comics" parts in the code and the database (having two "chains" meant twice the work each I had to modify something), changed the language I use to develop Ubooquity, and dozens of other minor changes. I will write a full change log for the release. :)