Acrobat Pro won't open Windows 11 Pro

I am running Adobe Acrobat Pro under an uptodate Windows 11 Pro machine, on continuous release, and have never had a problem in >10 years. In the last couple of days it became unresponsive, which cleared if I waited. However, this got worse, when I used some pretty large PDF's. On researching the point, I disabled Protected Mode and Advanced Security. Shortly thereafter, the App just showed a dark screen with a dash, a square and a X at the top. I've tried these steps, all to no avail: repair; uninstall and reinstall; uninstall, run cleaner and reinstall. The icon appears in my Taskbar, and if I right click it a list of recent files is there. If I try to run one, I get an error that 'there was an error opening this document. An internal error occurred.' Any ideas on a fix would be welcome and appreciated. 

Ian