Error: 500 Internal Server Error

I have this error in PaperUI in System/Things/Items. Searching through the forums seems to indicate that this is due to incorrect item definitions. I commented out all the items and enabled them one by one to figure out which one was causing the problem. The error did not come back after enabling all the items (all items seemed ok). But then suddenly after a day the error came back.

Is there any way to find out why i have this error message?

I am running 2.1 with the following addon snapshots:

# ls -l -1
org.openhab.binding.harmonyhub-2.2.0-SNAPSHOT.jar (
org.openhab.binding.zigbee-2.2.0.jar (
org.openhab.binding.zwave-2.2.0.jar (