I searched the Fibaro forum for this problem with Walli switches, but did not find anything about it.
But I know a case where a user had the same problem with several Fibaro RS3. The user had even contacted Fibaro, but without a satisfying result.
These are the corresponding posts in the Homey and Fibaro forum:
So my recommendation is to contact the dealer and/or Fibaro.
So I did, I got 2 new Walli controllers back, put off the power, installed the switches and they are operational again, but guess what. Due to the power cat, another Walli controller has the same issue. I found out on the Fibaro forum that people had the same issue and they solved by lowering the sync frequency from 1500 to e.g. 300. But where can I lower the sync frequency? Is that a Homey setting or a setting within the Fibaro Walli Controller?