I think I have it fixed now.
After unplugging the MySensors gateway (ttyUSB1) and plugging it back on again the error message at startup was gone.
After clearing the Chrome browser cache the Pimatic interface still does not work but in Firefox, IE11 and Edge it is working.
Still strange because I always used Chrome for Pimatic.
When scrolling down in this text dump I see a logon part at the end.
Submitting my credentials then show me a popup where again I have to submit my credentials and then the user interface is back.
In the url bar of Chrome I see my credentials (I replaced them with ******* below):
http://192.168.1.75/?username=*******&password=*******&rememberMe=on
Strange behaviour. Something with Chrome but how to solve?