Solution:

In the [Machine: Paper Feed/ Output] group of the [Adjustment Settings for Operators] menu, set 0304: [Correct Output Paper Curl] to "U Curl Correction Level".

If "Off" is selected, change the value to "Small". If "Small" is selected, change the value to "Large".

If you cannot remove back curls, print with the printed side face up for single-sided printing.