Thanks for reply!

So it's more a feature than a mistake in my case - lol. So I'm sorry, that my posting wasn't more helpful to solve the problem for those, who definitifely have more trouble than me.
I personally can live with the one mouse-click to make the error-message dissapearing. Finally the liberary loads within seconds - in the past it lasts up to one minute. Under the line that's much better than before, even with the unnessessarty fault-message.

However, if there's a solution to stop this message when working offline, let me know, please!