Jim, thanks for all your effort that went into Pan X and the state it is in now. For my simple requirements it is already a viable DB, especially with the forum and the experts available there in emergencies.
There is one recurring nuisance, however, which pops up unpredictably, now less often than before, but I am still occasionally faced with the text above in a pop-up window. Last time it happened, I was afraid to lose several days of work. When this happens I have to force quit PanX but duplicating the file in question and opening the copy with PanX usually solved the problem. Not nice but it used to work. In this last case it did not.
Finally, in desperation, I tried "alt-save" (= save as) and the file was saved (not duplicated), with a slightly different icon. After that it worked again.
What worries me is the unpredictability of this error. I don't know why it occurs, I can not reproduce it at will, it just happens occasionally. Usually, when I want to save modifications to a file which required a lot of detailed work.
Do you have an explanation?
Have latest PanX, latest Sierra.