Thank you, LeonidS! I think, I found the problematic error, LeonidS (by the way, I'm not allowed to tag your name as a basic member here):

DevTools failed to load SourceMap: Could not load content for ht***://community.gruendungs.team/marka.min.js.map;: HTTP error: status code 404, net::ERR_HTTP_RESPONSE_CODE_FAILURE

I get it 4 times by loading the page. A 404 http error means that there is no such page, right? Does it mean that the js tried to find content to put out on the page, but couldn't find any? I cleared all cashes, but those error lines remain.

As to the other error notifications: I had 2 add new content blocks at that homepage, in order to place the blocks differently for large displays and small ones. I deactivated one now, which helped getting rid of the DOM error message. I tried to deactivate Google Maps by deleting the API code in General Settings. The warning I got that for is no big deal.

Appreciate your support very much. Thank you!