The cause of the error is a conflict with an old record for this employee in the information register "Employee Personal Accounts for Payroll Projects". The system does not allow changing the existing record because there is already another record for the same key fields.
How to fix:
1. Find the old record for the employee in the information register "Employee Personal Accounts for Payroll Projects".
2. Delete this old record completely.
3. Create a new record with the required payroll account — specifically as a new record, not by editing (overwriting) the old one.
After that, the error when changing the employee's payroll account no longer occurs.
If you have difficulty finding the required record in the register or verifying that the old record has indeed been deleted rather than just edited — note that it is precisely the attempt to "overwrite" the existing record on top of the old one that causes this key field conflict. The correct order is to first delete, then create a new record.