Ok, sorry. This is because I have unintentionally mixed up things here:
there have been two overlapping problems:
- The conversion warning causing because of a wrong value in the item also causing the ‘Err’ display in Basic UI.
=> I have solved this by setting a dummy value at boot time
ShPl_S1pow.postUpdate("17.0867 W")
- When the shelly device is not powered on at boot time (when the binding starts) it does somehow not initialize the channels for this device correctly. Because when I turn on the power and switch the relay the device sends its power usage to the channel but OpenHAB does not receive any updates as the karaf log show not updates to the appropriate item:
Number:Power ShPl_S1pow "Shelly S1 GH Pump Energy [%.2f %unit%]" (gShelly) {channel="shelly:shellyplugs:######:meter#currentWatts"}
I see no updates for item ShPl_S1pow after I have turned the device on …
Hence my question is if and how the binding is recognizing devices that get turned on after boot time to enable their channels to receive the data they are sending?
Did I explain this in an understandable way?
And I guess I finally found the community topic of the shelly-binding creator and posted my question about the device handling there 
So this topic can be considered as solved … I will write the solution into a separate reply and mark it as the solution.