I have a very similar repeated "bug" in MS Project.
When I try to "Save" or Save As" my working (open) **** MS Project file, after several hours work, I get blocked with the following error message (in pop-up box with red circle with white cross symbol inside): "The file "[xxx]" cannot be opened", even though the file is already open!
I lose all my work from the last "save" if I close the open file - so no new work is saved, even in the *.BAK back-up file I have set up automatically in MS Project, because this file saves no more work than the original file.
This error message and block still occurs if I try to "Save As" the file under a different name or in a different location or if I attempt to "Save As" another version of MS Project.
If I close the file, I lose all the work done on the file from the time I have opened the file in the morning and try to safeguard the file several hours later.
I have even set-up MS Project to automatically create a back-up file (*.BAK), but this only saves the work done up to the same time as the original file.
So, a serious fault is apparent in MS Project that does not permit files to be saved after being worked on.
The only way round I have to cut+paste all the tasks (rows) - can be several hundred lines/rows - into a new 'virgin' MS Project file, but this means I have to then re-set all my settings and formatting in this new file, wasting additional time each day (when I want to save new work on the same file).
Any possible solutions would be very welcome!
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MS Project is refusing to actually save the changes I make for status. If I try to simply make the changes then save and close, when I close it asks me if I would like to save the changes again, and promptly does nothing. When I try to 'save as' to force it to keep the changes, it simply updates the name of the old file with the new name and still dumps the changes.
The file isn't 'read only, there are no error messages popping up, it simply will not allow me to keep changes.
When I look at it in 'Apps and Features', this is how it presents:

Has anyone else had this problem, and is there any fix?