Hello,
after I have contacted Arduino support, they have informed:
Its a known issue of hangs because with the current hardware and software revision, the minimum reachable current consumption is 20mA (the level transistor consumes ~10mA each in every condition). However, we are working to achieve the best performances.
and also:
The MKR GSM 1400 board hangs time to time and its a known issue. I am talking generally on the MKR GSM 1400 board issue.
We are waiting for the answer to this issue.
they will do a refund for the products.
I hope to be solved quickly, so there is a new hardware version for the MKR GSM 1400!