Thanks for your reply!
1) I have two SIM800L’s available for testing, the small red 800L core board and the RoHS SIM800L EVB 2.0. I guess they are both plane boards.
2) I tried different power supplies. I tried some battery stacks and a Meanwell HDR-15-5, 2.4 amps with a 100N capacitor. Both SIM-modules mentioned above are running steady now on the MINI.
3) Which pins on the X2 do you exactly mean for UART communication? I tried pin 16 (TXD2) and 17 (RXD2) and pin 18 and 19 on the MAXI, neither of them were succesfull.