I really dont know what might have caused this.
I was able to download the newest updates, but had an error installing it. Again a bit later, the download rate dropped again towards 0. Its a LogiLink WL0084B adapter.
Because of the wifi issue (as I said, this came out of nowhere) and the error during updating, I simply reinstalled RC2 on the SD card. Currently I’m having a different adapter attached, which seems to work better so far (except that OSMC seems to forget the wifi credentials everytime it got disconnected).