First of all , Steve, thanks for your reply, I think your explanation is pretty close to what actually happend.
Meanwhile we have compacted the database and it's size went down from 13 GB to only 4GB ! And, most important, afterwards I could and still can get the total amount of docs in the database via db.GetAlldocument.count.
This made me a bit curious about what could have been the reason for such a big difference in size as there were no data losses at all. I found out, that this DB is used as a backup DB for a ticket DB. Every day new tickets get stored in it and ... everyday those older than 1 year - get deleted. So obviously this DB must have been used for a while without compacting and I assume that all those deletion stubs must have added up to a huge amount and have caused this condition (my theory). But its strange that the cluster servers do not report a db with such extreme conditions.
I really would appreciate if IBM could give us their official error explanation(s) ...