I had the same problem. It appears that if you completed the inputs for Form 2210 prior to the 2/26/21 update, then a "ghost" of the 2nd period withholding input is not removed from the worksheet when you re-enter your withholding for the three periods of 2020. The TurboTax error checking picks up that ghost and adds it in, thus creating the error reported.
I fixed it by editing the worksheet that is shown on the page where the error is reported. Trying zeroing out the "ghost" period (should be the second one).
