14.4.5 LTS upgrade problem on Fujitu SIEMENS Laptop

[Nino Novak]

Hi,

I just tried to automatically upgrade my daughter's ESPRIMO Mobile 
U9200, which hasn't been used for about 2 years, to a recent Kubuntu 
version. The old system offered upgrade to 14.4.5 LTS (IIRC).

Download went smooth, but after automatic reboot the installation hangs, 
screen stays black, load goes to 1.29, fan blows loudly.

Changing to tty1 and login successful:

top shows that NetworkManager consumes ~42% up to 97.8% of cpu.
sometimes apport runs, sometimes plymouthd - no other active processes

dmesg has hundreds of entries like this:

network-manager main process killed b ABRT signal
network-manager main process ended, respawning

How to proceed?

(I'm not skilled with kubuntu, but had been using openSUSE for several 
years, so at least have some basic linux knowledge)

Thanks for any hints.

Nino