RE: Voucher xxx is already used as of date xxx
Hi Jonathan,
I totally agree with Ludwig and André answers.
I had this issue before. I can suggest that you check number of sequence setup (Segments and General) which is used against this payment journal "Voucher series" (GL > setup > Journals > Journal names)
When you open the accurate number of sequence, you can check "Status list" and if there is a line there, delete it (i.e. select the line and Ctrl + F9).
On the 'General' tab check if the "Continuous" is marked and then try to enter a number on "Next" field which is not already used.
You find unused voucher number (i.e. based on the number of sequence setup that is used for the payment journal) by checking the the GL > Inquiries > Voucher transactions (also, Check the payment Journal against latest posted and open journals' voucher to find a unused voucher and then check it on Voucher transactions to be sure about it).
I hope it helps,
Regards,
Alireza