Nope, it's not. I have a three tier system:
1. First the Thompson 585 DSL modem which is set to bridge mode, so...
2: ...the Linux M0n0wall firewall controls the DSL connection. And I do of course have full control over the firewall in the M0n0wall, and even turning that completely off doesn't help. The M0n0wall connects...
3. ...to a Windows 2003 Server that is the main house server, and the firewall on RAS is disabled. Then a WLAN router (Netgear with DD-WRT Linux firmware) is connected to the server, and it gives wifi access to everything mobile and portable in the house. That's connected as a pure access point, with the WAN port configured as a LAN port and the firewall turned off.
That means that disabling the firewall protection on the M0n0wall turns off all kinds of firewall protection in the house, and I have tried that. I even tried connecting another wifi router as an access point directly to the M0n0wall, bypassing the Windows Server, and that did not change anything either. So I'm very sure this is a problem at my ISP.