It seemed that for some reason I got connected to the WiFi access point in the basement and not to the repeater. You could use the excellent wavemon tool to monitor the signal strength of all available access points. (
sudo apt-get install wavemon && sudo wavemon)
The fix was really easy even if I had to use the mouse for it ;-) Click on your WiFi symbol and choose "Edit Connections ...". Then choose your WiFi connection and press the "Edit" button. Click on the "WiFi" tab. In the BSSID dropbox choose the one you want to connect with (The one with the strongest signal shown in wavemon). Don't forget to press "Save". That's it.