Hi @viktor_laitinen,
Could you please try again with a LiPo battery connected, from the MKR GSM 1400 product page:
During cellular transmissions the peak current required by the board will exceed 500mA. This is in excess of what can sourced by a standard USB port, so it is MANDATORY to have a 1500 mAh or higher LiPo battery plugged all the time, the current provided by the USB port will be supplmented by the battery.
I repeat from the post above:
Could you please provide a full minimal sketch that can be run to reproduce this? Without this, it is very difficult for us to try to reproduce the issue.