Thank you all for your suggestions. I have solved the problem now, the culprit seemed to be a bit of everything. I hope this could help anyone else having problems using Win10 with ZK.
The most perplexing thing was that even after file access rights had been righted I kept having unexplained access denials, without anything indicating that it was actually the anti-virus software (Avast) blocking access. After some experimentation I found out that Spring's v.529 has been reported as safe but later versions were not, thereby misdirecting troubleshooting efforts into the different engine versions. I have actually disabled file reputation services, but it was merrily enabled in some update without questioning the user. The error indications that came only from the operating system also meant that I misdirected my efforts towards Win10 even after file access issues had been corrected.
Some distilled wisdom in no particular order:
1) Run ZK as admin or with an admin user account
2) Check anti-virus software's settings very thoroughly especially if updating from Win7
3) Check ZK's folder permissions to have full access to files in the account you run ZK with
Very special thanks to the community and especially the very patient
GoogleFrog , sorry to bother you so much with engine versions as the main fault unlikely laid there.