I get this error intermittently. Yesterday I worked on two different projects, saved them each before leaving at the end of the day, but left them open on my desktop. Today, after making a few cartographic cahnges, I get the Project Save Error message: Failed to save project General Reference to G:\GIS Projects\General Reference\General Reference.aprx . Argh.
They do have spaces in their names, they were created in older versions of Pro, but have been recently accessed and saved, so that shouldn't be an issue. Last time this happened (within the past few months) I was able to do a Save As in a new location (or just a new name, can't recall) and it was fine. Today it won't even let me do a Save As: new location, new name, network or local. I suspect it has something to do with leaving the project open overnight. They are both originally saved on a network drive. I am at 2.2.3 on Windows 7. One is connected to SDE (though no active layers in the open Map and Layout), the other is not. Again, leaving open overnight after no issues with saving the previous day is the common variable. Very very frustrating.