this particular case was Windows 10 - RetirementView 2020.3b
For your learning to help others, I will detail what the problem was.
As you suspected, it was the Microsoft Virus checker (the one that is built into Windows 10 by default and is called "Windows Defender"). Specifically, it was blocking any writing to the directory and to the key file Defaults.rp8:
Documents>Torrid Technologies>Retirement Savings Planner > Defaults.rp8
I found the procedure to unprotect that folder: https://answers.microsoft.com/en-us/protect/forum/all/how-do-i-remove-a-folder-from-the-list-of/7406b3f1-bc16-4b8d-9e3c-afccbfd632b0
But after I applied that, I also had to explicitly "allow an application" to access the folder; so I specifically had to allow "RetirementView.exe" to access the folder.
After these two changes; it is now working and saving my changes.