Hitron BRG-35503 - Firmwareupgrade

We have received a newer firmware for a rather old Hitron BRG-35503 4x4 Cablemodem.
I've set alle the required configuration for upgrading the cm.
But it doesn't start to download any firmware from our tftp server.
Any hint would be helpful.

After a modemreboot I can read all necessary informations by snmp:
Name/OID: docsDevSwServer.0; Value (IpAddress): correct
Name/OID: docsDevSwFilename.0; Value (OctetString): correct
Name/OID: docsDevSwAdminStatus.0; Value (Integer): allowProvisioningUpgrade (2)

Even the modem-log has no information.
CVC could be wrong, but the modem should first start downloading.
So, anybody a good idea?