RAK8212: Issue with save() freezing up the system

I am currently playing around with a RAK8212 and wrote a module to interface with the Quectel BG96 that is aimed at Narrowband IoT (NB-IoT) communication. I tried my best, but I am a newbie and possibly the code quality is bad ...

But the actual issue is that when I call "save();" the RAK8212 freezes up and I need to press the reset button to get it back into live.

I would appreciate some hints what I am doing wrong or how the issue can be solved.

I have placed the code into a github repository at https://github.com/wklenk/rak8212-esprui­no-nb-iot

Output:

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>Setting up external hardware. Please wait ...
BME280 wiring set up.
Quectel BG96 wiring set up.
>save();
=undefined
Compacting Flash...
Calculating Size...