It works!  I tried the new version 2.1.1 and you've fixed it, atleast for the books I have tried it with :).  Thank you very much, you will be receiving a donation from me very soon...

At the moment my comics all work...and in fact some of the books work as well.  Thanks for reproducing, I'm very excited to start using Ubooquity more and more :).

This bug is the only issue currently stopping me from my whole extended family from using the tool as well as my work (which could expand to up to several hundred users).  Please fix this ASAP as Ubooquity has tons of potential already even with the bug!  I will most definitely want to donate some money once this bug is fixed, as there is no web-based tool that is nearly as easy to setup and use as Ubooquity.

I am seeing the exact same issue.  I have also tried multiple browsers and I get the error on quite a few books, while some books I do not get the error.  I am hosting from Windows 8.1 with Java 8 in interactive mode as well.