Hi,
thank you for your answer! I am very thankfull for your insights and ideas and i think i know what the problem is. The pump is apparently running but i didn´t felt anything running in the pipe so i poked the pump a little bit and afterwards i could feel something flowing and the temperature where sinking to about 46°C
I test it and it seems like it does flow but only a little bit and not enought to cool it efficiently.
To answer your questions
I had just started the system, i did nothing at that time.
Nonetheless everything seems a little bit weird to me and i will most likely buy a new cooler tomorrow.