Error message “Couldn't submit the process” on the assignment page for local and global transfer

We use DFFs on the assignment page. We use drop-down menus, text fields, and checkboxes.

When we click on “Submit” in the Local and Global Transfer transaction, we get the error message “Couldn't submit the process.” Below that, all of our checkbox flexfields are listed with the text “[Flexfield name] N is not in the list of values.”
However, we have entered Yes and No as the value set, and Yes and No are also stored for the Checked/Unchecked value.
Initially, we had Y and N there, but we changed this a year ago and never had any problems.