bah, I hate trying to solve issues I don't have myself
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 You probably will have to re-download the jnlp file. If the problem persists, can you post your current set-up (java version, x32 - x64 bit, operating system)? Hopefully it'll allow me to actually reproduce the error.
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.