I found a workaround for my problems now, the problems described above were caused by SIM2. During the last days I suddenly also had connection issues (the same ones as described here Since update: SIM-cards connection problem). I took out SIM2, and suddenly everything was fine, no problems anymore!
How can I solve my SIM issues? I use my second SIM card when I’m traveling for data (it’s a UK SIM). Any idea @AnotherElk ?