One of the various 'changer' tools allowed for a successful change of the Android_ID (not S/N, mind you). Of course with no impact on the Navigon app, as it really relies on the S/N, which is oddly frozen on my device.
I do have the TWRP backup, and besides the root everything else is 'stock' as there is very little activity around this fake tablet from Voyo (sold as 6.0 with 1.5GHz and 5Ah, in reality a crappy API 22 device with 1.3GHz and 4Ah battery). Russians hacked it somehow and even tinker with a Android 7.0 update, but I really do not want to mess with it that much. 250 apps are running just fine, just no Navigon at all.

Maybe I can find some help at XDA where I will officially ask for a method to fix a "frozen" serial number for good, if on the fly each time I boot this thing, whatever.
It is my utter belief that a unique S/N will cure my problem, and most likely future similar issues as well.
Until then I am swallowing pills...
Thanks for the continued support and ideas.