I think I have found the cause of the problem. I reconnected the Walrus Audio Canvas and turned on the receive clock, so the BPM is being driven externally again. Now I first removed the MIDI cable from the Helix Stadium, and after that, the BPM tap (footswitch 12 Stadium) worked again. I probably turned off the Stadium last time with the MIDI cable from the external clock still plugged in, causing it to lock. The tap tempo is working again now!