Unregistered node was not found based on its id

I’m seeing a lot of

Exception handling request to /: java.lang.IllegalStateException: Unregistered node was not found based on its id. The tree is most likely corrupted.

in the server log of our internet-open production server. I’ve never seen it in my development machine or test server. Is there some known hacking-attempt that would look like this?

Thanks in advance,
Nik