azslow3

Rob[atSound-Rehab]
Thanks. I have already reset the ownership to my account. Unfortunately did not fix it ...

Have you checked effective rights or just "set the ownership"?
 

Yes in fact I did. Both set to the actual DAW user but still no access for Sonar to save to the folder (manually creating e.g. a text file worked fine there)

Apart from that win10 has been ok for me on the DAW. Not apparently better than win 7 but not worse either

What I dislike though is that all apps see so many updates in such short cycles ... This gives you always the chance to run into a problem that was not there before, even when updating only every 2nd or 3rd month...

Before I gave up, I would re-name your troublesome folder. Then re-create a new save folder from within the program, then reload the affected project from the re-named old folder and save it to the new folder, or just copy the old folder contents if that works.
 
Check that effective permissions is the same for all the troublesome files not just the folder, as there can be a discordance between file and folder permissions. 
 
Perhaps there is a stuck leftover inheritance issue? Nominally inherited permissions should show as grayed out users in the list, but your system is not behaving nominally. Try disabling permissions inheritance in the troublesome folder. Alternatively go to the top level folder (parent) for your troublesome folder, check that it has the required permissions and then try to reset the child folders to that permissions set by choosing to "Replace all child object permissions with inheritable permissions from this object." in the parent folder to see if that will reset the permissions in your troublesome folder. 
 
Or stand on one leg and scream like a chicken.