I've solved my issue by sheer luck and desperation. I decided to poke at the provisioning process again and found this older document named "intel_mbexsettings_v1.pdf", which I downloaded from here: ftp://download.intel.com/support/motherboards/desktop/sb
In the document several options for unprovisioning a system are discussed, among them a hard reset by clearing the CMOS and removal of the board battery. Afterwards I had to repeat the setup routine again and lo, the system allowed me to log into the web interface.
Well, that was unnecessary hard but I'm glad I've come to this point.