Moral of the story – read the logs
This one had us stumped for a while so I though I would post it.
We had an inconsistent error on an XPage application where it threw a save error even though the Author fields  were correct.
The error was

Exception occurred calling method

After much head scratching followed the advice in log.nsf

please consult error-log-0.xml located in /local/notesdata/domino/workspace/logs

whereupon we discovered the message

Could not save the document 5386 NotesException: Notes error: The document is already locked by someone else.

So document locking was turned on which is not really much use in an XPage application ( we rolled our own application scope based scheme )
So I know it is a pain to connect to the server to get the log documents but it is worthwhile. I wonder could we write an XPage app to recover them ? 🙂