I’m happy to report that with the help of Steinberg support this problem seems to have been solved.
It appears all three of my E-Licenser keys are quite old (early 2000’s) and they ALL failed simultaneously - in trying to solve the problem I transferred my N8 license from one to the next, and they all showed the same errors on starting Nuendo.
I bought a new E-Licenser and moved the licenses onto it and so far all is well, no more errors. As a bonus. Nuendo boots up noticeably quicker - it may be that the old keys (which have slower processors) have been struggling for some time.
The moral seems to be, get a fresh key every so often, specially if you start getting E-licenser errors.