But still, I would have expected that Electrum let's the user know that there is a problem and, therefore, the shown values can't be trusted. Instead, as shown in the screenshot of my last post, everything seems to be fine from the GUI user's perspective (green ball in the lower right corner and no error message).
I agree with you on this. Electrum messages usually don't let you know what's going wrong. Electrum usually produce a generic message when something is not right and you have to figure out what is the cause of the problem.
But in this particular case, it's obvious that Electrum didn't finish syncing or something went wrong. On the to bar menu it's showing: "imported, nur lesen " and when translated it means "imported, no read". So it's obvious there is something wrong. And it didn't show any wrong values, it didn't show any values at all!