Sorry for not adding the log files, I also had troubles connecting via SMB at that time.
Today I finally have time to fix my Raspberry, but seems like there is no fixing necessary! I saw the time was correct and connecting to the repositories works again!
So the question why it did not work back then remains, since I did nothing except keeping my Raspberry shutdown for a week.
I'd also guess that it's the time sync problem that prevents you from connecting to the repositories - if I happen to stumble over that problem again, I'll upload the log files. Currently there is only todays log files on the SD Card 