Error 633

A week or so back, I'd finished using my broadband connection and was gonna
disconnect it as normal but found the status icon in the system tray had
disappeared - eventhough the connection was fully operational. So I just
shut the machine down. Next time I started it up, I got an error 633 "the
modem (or other connecting device) is already in use or not configured
properly". Happened again last night.

Modem is the usual BT Openworld SpeedTouch 330.

Any ideas whats going on?

TIA Midge