Error: Symbol Not Found Error when installing WF

What do you see in the stack trace in the log file?  That will tell you where the crash occurred.

There is no log file.

Only one CIQ_LOG.BAK without any stack traces

On device in folder //GARMIN/Apps/LOGS I have only one file CIQ_LOG.BAK

The .BAK vs .YML seems to occur when the log is uploaded to garmin for ERA.

Do you have the background permission set for your watch face?  If you don't needed it and aren't set up for a background service, remove it, as otherwise, onAppInstall/onAppUpdate logic will try to load your entire WF as a background service.

This error does not appear in ERA.
The background mode permission is set and it is needed for connection.
is the ad enough (:background) to limit the parts of the code available in the background?

You don't want the annotation unless something is needed by the background, as the background has limited memory.  When your watch face does run, does the background run and the data gets updated?

So if you leave the WF running and do nothing else with the watch, the weather data updates as it should?

yes. once an hour with a successful update and once every 10 minutes if there is any error

I start the timer like this:

        if (Properties.getValue("WeatherProvider") != 0 || !SC.doTimer()) { return; }
        if (System has :ServiceDelegate && Background has :registerForTemporalEvent) {
            var lastTime = Background.getLastTemporalEventTime();
            var weatherUpdated = Storage.getValue("weatherUpdated");
            if (lastTime == null) { Background.registerForTemporalEvent(Time.now()); }
            else if (weatherUpdated == null || weatherUpdated < Time.now().value() - 86400) { Background.registerForTemporalEvent(lastTime.add(new Toybox.Time.Duration(600))); } // 10 minutes from last update
            else { Background.registerForTemporalEvent(lastTime.add(new Toybox.Time.Duration(3600))); } // 1 hour from last update
        }

Once you start the temporal event, no need to keep starting it based on a timer.  And no need for  Background.registerForTemporalEvent(Time.now()); 

in getInitalView, just do:

    	if(Toybox.System has :ServiceDelegate) {
    		Background.registerForTemporalEvent(new Time.Duration(10 * 60));
    	}

for example