I am getting repeated errors from a GW Object API Application:

Error while processing message <ID> of type GW.MESSAGE.MAIL.Internet: Could
not find the folder object

'Could not find the folder object' appears to be a GroupWise error. I can
find no reference to it in the documentation. A Google search only returned
a couple of hits from a Blackberry / GroupWise connector suggesting that the
issue was due to an account with 5000+ folders, but that does not appear to
be the case here.

What can cause the error?
Is it related to the specific message class?