Solved it, but what a nightmare....
At first I followed Stephans advice: uninstalled Java and NetBeans (many versions, collected over the past few years), and cleaned up my system variables.
Then installation of Java 8.131 went smooth (wel I think it did). However, with NetBeans I kept getting this error:
(by the way: Sylvia is my wife, it is her laptop officially)
It was maddening, and searching the internet for this 'canonized' was without any success.
It was only after I made a screenshot of this error and tried to save it via Windows Paint, that I got this "io exception: no more files" too that I started searching for this error.
And that helped. See for instance:
this site
It seems to be related to what is called the "Asus Data Security Manager". My (wife's) laptop is indeed an Asus, and I followed what that site said: remove this ADSM via the windows add/remove software, and what a relieve: I could suddenly save this error-picture and, best of all, my NetBeans is installed and running!
I really don't know why this error suddenly popped up, or how long it has been going on. Maybe since the latest Windows update from a few days ago?
I hope that this reply helps someone facing the same problem.
And Stephan, a
cow for all your help!