There is a feature that we sometimes use on our Cisco 10k CMTS that maps a modems CPE traffic to a VLAN. The command we use is as follows: 'cable dot1q-vc-map xxxx.xxxx.xxxx GigabitEthernet3/1/0 507'. With this command in place cable modem will not register. It gets to init(o), it looks like the TFTP transaction for the Docsis config file is completed as there is two way community with the TFTP server from the modem. If the modem is registered when the command is entered to the CMTS everything works until the modem reboots, at which time it fails to register.
What modem brand / model / firmware ?
Is the modem L2VPN compliant? ("show cable modem x.x.x.x verbose | incl L2VPN")
Do you use a different config file for these L2VPN customers compared to normal internet customers? If so, does the L2VPN config file have any "special" config in it (eg DUT filtering)
What does the modem event log say (browser to the GUI, or snmpwalk it)
If you get really desperate, you can try enable some debugging on the CMTS, perhaps :
debug cable mac-address x.x.x.x
debug cable reg
debug cable tvls