@DennisT
I would recommend you to set debug message levels to the highest level and watch out for suspicious log entries. You should be able to see a similar log message like this to make sure that the serial path(s) was detected by the OS aswell:
But notice that debug logs are gone when you refresh the Website!
Switching between Rpi0 and Rpi3 is intended to work AFAIK, thats probably the reason why the firmware path function was made. Here is the long story.