I've just completed a series of tests as follows -
1. I created a new project from my orchestration master, copied something across from the original project, and saved successfully. I closed and reopened the test, and made another edit successfully before saving and closing.
2. With the original project, I made an edit and selected 'save as' rather than 'save.' This saved successfully, and I was able to reopen, edit, and save normally. I was also able to do this using the 'save a copy as' option as well.
I'm not an expert on Logic or Mac OS, but it leaves me thinking the problem is with the original project files. Perhaps a backwards compatibility issue. The original project was created in June 2017 ...