Thanks, Bopper. Your solution didn't work on this model, but I did manage to solve the problem.
For the record: you can reset the PBX to its factory default settings once you've logged in with the 'basic' password. Just use the corresponding function in the top left menu of the standard Siemens Manager. This is strange, because you would think this option should only be available to 'advanced' users. Lucky for me, this wasn't the case.

Paul