I ended up doing as described in my earlier post; Downloaded and burned "East Europe V11 (8129)", inserted the DVD into the RNS, let the installation run for a minute or two and then aborted by ejecting the DVD. After that I inserted the previously burned "West Europe V11 (8117)" DVD which forced the corrupted maps to reinstall.
For the installation I made sure that the process could run uninterrupted from start to finish - i.e. no reboots - but like the last time it ran almost to completion before starting over. This happened three times. On the fourth attempt I let it run to about 80% and then rebooted it manually. It then started from where it was before the reboot and - coincidentally or not - completed without any errors. My route guidance now works as intended.
All good. Case closed!