You can see these information in the log file.
Look for lines like:

20140726 18:10:46 [pool-2-thread-6] INFO  com.ubooquity.a - User successfuly logged in. [login:Tom][ip:10.0.0.5]

to know when a user logs on, and

20140812 19:57:34 [pool-2-thread-5] INFO  com.ubooquity.f.d - Served file to Tom in 21 sec: Deadpool 014 

to know when a user downloads a file.

The error you get comes from images contained in your PDF files that are encoded using the JPEG2000 format.

JPEG2000 is an image compression format that is quite efficient but also very complex and that never really took off. Because it is complex and not really used, it has very poor support in applications and libraries.
In your case, the PDF file is decoded by Ubooquity using the JPedal PDF library. This library does not support JPEG2000 out of the box and has to rely on another external library: Java Advanced Imaging (JAI). The problem is that JAI is an old library that is not maintained anymore (abandoned since 2006). 

So in theory, depending on the architecture you're running Ubooquity on, you might be able to download the last JAI package, install it and add the JAI library to your classpath when running Ubooquity. But I doubt it will succedd.

Bottom line: Ubooquity does not support JPEG2000, sorry.

I had already explored the possibilty to add an epub reader in Ubooquity, but at the time none of the libraries I had tried (Monocle and Readk.it were working correctly).
I did not know epub.js. I'll test it and consider using it in the future. Thanks for the info !

I'll add it to my todo list.
(this sorting criterion is already available for comics ("Sort by path") but not for ebooks indeed)