I rolled over my 457b (my pretax contribs) to a Roth 457 in 2020. 1099R says "0" in box 2a for taxable amount. Is this a mistake? I thought it was all taxable.

A zero in box 2a would be a mistake unless all of the conversion came from after-tax contributions held in a separate sub-account there had been no investment gains in that sub-account.  A zero in box 2a would be correct if box 5 had the same amount as is in box 1.  Otherwise, you need to obtain a corrected Form 1099-R.

Thanks so much, I thought so!  Appreciate it.