[axlt2002]

Ok, this sounds really weird but if it works is good! Did you make the modifications directly to the files whitout reinstalling the add-on?

Yes, this is fastest way
I run addon and follow errors in logs, change one file and run again. Also with scheduled.

Ok, may be there have been some changes with respect to the previous versions of LibreELEC (even if it sounds really strange to me!). And I can not understand why the same issue was not experienced by djhifi...

About the scheduled time I dind't have that issue. Please note that such debug string actually appears when you change the schedule time and press the OK button in the settings. When the service is just launched at Kodi start, that strings does not appear in the log. I have tried it right now and the scheduled time is updated without having to deactivate/reactivate the add-on.

I checked this very strictly (few times). Scheduled work not start (8.1.1) after change, press OK, disable/enable in addon config and so on. Only deactivate/activate of addon gives scheduled work (and gives line log about settings this scheduled work). After addon activation there is PHP update, then window with info appears and after click this info -> log line about settings of schedule job shows. And only then scheduled work start at time.

Please note that any change to the scheduled time has effect (and sense) only after the PHP installation has been performed. As you described, it seems you tried to change the scheduled time before it. What it is strange (this LibreELEC 8.1.1 is going to make me crazy!!!!) is that the PHP installation should start automatically after having installed the add-on...did this happen or not? In fact, deactivating and reactivating the add-on is almost the same as reinstalling the add-on. Can you please try with a clean installation of the add-on with the debug log enabled? The PHP installation is expected to start after around 10 seconds...can you confirm? After that you can change the scheduled time as you prefer.