I found the problem. Apparently Lenovo Vantage (some kind of control center which came with the laptop) has a network boost option, which was activated every time I started a game. It was suppose to limit the bandwidth to any other services than the game. But instead of doing that, it also limited the bandwidth of the game. The strange thing is that I never changed a setting of the network boost, maybe something happened after an update. But I feel a bit stupid that I never checked it.
But thanks to everyone who tried to help!
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