After exhaustive research, I think the e-file rejection problem is at the IRS level. (Sorry for my frustration, TT.)
Solution: Using a zero for AGI worked for me. Since this AGI input is just for identity verification, it has no bearing on my tax return but allowed me to e-file without being rejected.
Here's a possible reason why some people need to enter a zero instead of their actual AGI: Even if the IRS has your 2019 AGI on record (which you see on an online tax transcript), if there were any processing delays for your 2019 return (such as a missing form that bumped the return out of its original processing stream, a mailed item, a late filing, or a delay on the IRS end, etc.) the record of your AGI and your 2019 filing might not be in synch.
