Exchange incr log backup promoted to full

Hallo,

I need some ideas: I have Exchange 2010 on Win2008R2 and NMM 2.4.1.

Backup schedule is level incr but if always backs up no only the logs

but also the databases.

What could be the reason? There is a level full backup as basis for incr.

Exchange is not in circular logging mode and the is no log gap.

regards

Bernd