Just wanted to throw another tidbit in since we're having the same problem with Stark:
If I set the coscript shouldKeepAround to false on our plugin's close handler, I can reliably run the plugin again without getting the error mentioned in the OP. The only problem is that Sketch will eventually crash if I do that. Sometimes it's immediate, other times it's up to 30 seconds later, but it will eventually crash with no error in the log.
If I don't set it to false, Sketch doesn't crash, but I'm unable to run that plugin handler again. Oddly enough, I can run any of the other 8 or so handlers, just not the same one twice.
Hope that helps a bit with diagnosing the problem.