Update on my 'an error has occurred':
- the tablet had only ONE SIM card installed (but two slots)
- when ONLY activating the mobile data (WLAN off) Navigon complains about 'server connection error', this indicates that it does not 'see' the internet??
- when activating WLAN and optionally also the SIM the known problem 'an error has occurred' is displayed
- the tablet shows a reasonable IMEI number when checking
- the associated version.xml file contains really odd and old data (rev 5.9, different IMEI etc)
- entering my real IMEI in the version.xml file makes no difference, the error remains when starting the app

What I have NOT tried yet:
- swapping the SIM card to the other card slot
- adding a second SIM into the tablet and try either/both at the same time
- using IMEI fake apps (but WHAT to fake, what IMEI should I enter, the wrong one from the version.xml file?)

What does this cryptic and bland 'an error has occurred' really mean? It is NOT a connection issue, rather a 'verification' issue as far as I can tell by its behavior.