Even with 2019 (which is generally more stable) I'm still experiencing this. Seems to be worse when various art brushes are used but that could be a red herring...
One workaround I've found is to leave the file open. Sometimes it just allows a save later (just hope it doesn't crash...)
The other thing that *seems* to work is to keep another older version of Animate installed: Open Animate 2018 or 2017, then open any file in that version of animate. Any other .fla file (not the problem one). Then close the file. Or make a new file and save it somewhere. Basically fiddle around with another file or two in an old version of Animate whilst leaving your current version with the problem save open... This seems to kick start your current version back into life and fix the permissions allowing it to save.
Another thing you can do if desperate is to open the older version of Animate and paste your assets into a new file. (You can't paste frames when doing this but assets on the stage should be ok.
This could be a Mac operating system thing but I've tried all the suggested fixes for permissions to no avail and it only seems to happen with Adobe software (I think it happened to me once in InDesign.)