These messages with error codes rarely occur. If they do - partner usually needs to report it via a Service Request and support to investigate and provide the cause.
When checking from back-end I see that error log throws 'nullpointerexception' when copying values of the PO onto credit memo.
I suspect that PO is missing one of the values that is mandatory for credit memo creation.
Is this happening for all POs or just one?
If for all, maybe I will need to find a working example and we need to compare what is missing in your PO that is causing this issue.