I wrote a sketch. Saved it is a different folder that is not the default. Tried to open it again … serious error (see image).
A second try to open file produced the same error message.
I opened the file with a text editor and pasted the code into a new sketch. The program runs. I saved it. Tried to open it and got the same serious error.
Saving the code to the default sketch folder avoids the serious error. (but who wants all their sketches in one folder?)
I’ve tried various things to try to reduce the seach space, but I have not figured it out. Other sketches have been saved in alternate folders without issue.