
Apparently this is an issue that has been cropping up with the latest java update.
It's supposed to protect users from people hijacking someone else's code (if you ask me, a ridiculous use-case) without any adjustments except for running it with different permissions.
i *hope* it's fixed now, but I wouldn't bet on it

Some further digging revealed that this is supposed to be caused by update 25 on version 7 java, which enforced the codebase entry in the jar manifest. Even my java is current again now, and it works for me. I only upgraded after "fixing" the error, so I hope it was thanks to that fix and not due to some other random incidents.