[GNC] "error parsing file" after upgrading from 2.6.x to 3.1

[Brian Buchalter]

Forgive me if this is a duplicate question, but I could not find any recent
results.

I am a macOS user and longtime Gnucash 2.6.x user. I wanted to try out the
new 3.1 release, but when I attempt to open my .gnucash file, I am told
there was an error parsing the file. No other information is given and the
file does not open.

Can anyone guide me in troubleshooting this please? If it's happening to
me, I imagine it's happening to others.

Thanks!

Brian