HI!
Using back-mdb there's one thing which could cause trouble: One has to configure olcDbMaxSize which cannot be changed later.
So when monitoring OpenLDAP with back-mdb it would be interesting to see how much space of configured olcDbMaxSize is already occupied.
Does it make sense to compare the real file size shown by du -h /path/to/data.mdb to configured olcDbMaxSize?
Ciao, Michael.