(ABR 11.5) Full backup instead of Diff

Hi.
I am using ABR11.5 commandline on a Windows 2012 Server to backup files.

When I start full backup from commandline it works fine.
The Problem is the differential backup. Instead of doing a differential backup it does a full backup.

Iam using following commands:

FULL:
acrocmd backup file --include=\\server\share --loc=\\storage\folder --arc=vault.tib --backuptype=full

DIFF:
acrocmd backup file --include=\\server\share --loc=\\storage\folder --arc=vault-diff.tib --backuptype=differential

TIA, Dejan