Thats's not the full info. According to this the required System for Acrobat DC Pro, for example, contains Windows 7 and for the question What should I do if I can't upgrade my operating system? this page says If you can't upgrade your operating system right now, you can continue to use the currently installed version of your Creative Cloud apps for compatibility until you can upgrade. Which is now not the case and contrary to your answer.

Thank you very much for the hint to the ICS-Format. Yes, it is the reoccurring wich makes the trouble using the CSV. Going to test ICS (iCal) now.

Is Outlook able to import ICS to a specific calendar ?

That would be possible, but actually, the issue seems to not occur each time after a boot - so it looks like it is a random behaviour. And that makes it a lot more difficult to find. Another idea: is it possible that the call of an update process / routine and the included deinstallation of the old version is the root of that behaviour? I'll have an eye on that as soon as the next update will be available.

Thank you, anyway for your idea. Maybe it's time for reinstall or switch to Win10.

@HelpingHand Do you know, where to get wpr.exe and wpa.exe for windows 7?

@HelpingHand : It looks like my assumption wasn't correct. In Safe mode I didn't see this behaviour and after some restarts, the behaviour couldn't be reproduced, at least not by starting and stopping an any application. But I had the behaviour several times. Do you have any ideas, how to determine the driver blocking the *.exe if it occurs again?

The type of the handle is 'file'. So I expect, in Safe mode the same behaviour. But I'll test later.