PSE 10 gives an error message when opening the Editor

I have been  happily running Photoshop Elements 10 under OS X Yosemite (10.10.5) on my 2015 iMac until today. Now I can start the program but when I try to open the editor I get the following error message;:

"There was an error opening this document. This file is damaged and could not be opened."

This comes up even before I have tried to open a photo.  I can ignore it and open a photo, but not all functions are available, e.g. I can resize the photo in inches but not in pixels.

When this happened some year ago under PSE 3, I was told to delete the PSE plist, paths and settings files in my Library. It worked then but didn't help tonight.  Any advice/suggestions/comments will be much appreciated.