Working as intended now - pure awesomeness!

Yes, that did it! The error is gone. It now gets stuck on certain books (scanning halts, no error) but at least it runs. Baby steps! Cheers for the help!

This is the command I start it with:

/var/packages/Java8/target/j2sdk-image/bin/java -Dsun.java2d.cmm=sun.java2d.cmm.kcms.KcmsServiceProvider -Dfile.encoding=UTF-8 -Xmx128M -jar /var/packages/Ubooquity/Ubooquity.jar -port 2202 -webadmin -headless -workdir "/var/packages/Ubooquity/"

I've tried 128, 256, 512 and no max memory and deleted and reinstalled both Java and Ubooquity.

I get the same error unfortunately.. :(