So this one was really really odd.
After nearly one whole day debugging I was able to solve the issue.
It has nothing to do with jRuby, nor with the active-record Adapter (you get a very similar error if you push this to heroku with a ruby-Runtime rather than a jruby-Runtime).
The issues was caused by this line in one of my models
validates_presence_of :terms_of_service
After commenting it out and pushing it to heroku again everything is fine.
Odd...