files don't open after restart

The problem seems to have spontaneously resolved itself, since now all pdf files that I've tried open and can be used without the error message appearing. If anyone ever reads these posts and needs a quick solution, you might want to try an alternative to adobe acrobat: SumatraPDF - see https://sourceforge.net/software/compare/Acrobat-Reader-DC-vs-Sumatra-PDF/

The problem still occurs sometime, on other occasions the files open. No answers have appeared from the community or any expert. I got a feedback question from the adobe community, asking if my own replies had solved the problem (!), and providing no way to answer the question. In the meantime I see someone else has the same problem. I'm thoroughly fed up with Adobe.

The problem appears to have spontaneously resolved itself, since the error message no longer appears when I open a pdf file. I was disappointed not to get any feedback to my post, in particular from adobe experts, and was about to switch to an alternative programme.

I also tried closing all applications and Help > Repair Installation in Adobe Acrobat, but that didn't work.