"But 6 minutes later i could reconnect and saw that the time for my opponent almost run out.
He then lost due to time rather than checkmate so ist was definitely a Server Problem for both sites"
This was my hope, too. But in a similar situation I suddenly lost because I "abandoned the game". I never ever did that. Why should I? I had plenty of time and could mate the opponent in a few moves.
Same here. Three times only in the last two days. All other apps /sites are working, router/internet is fine. And it only happened when I was in a winning position and could mate in a few moves. Frustrating and first of all suspicious.