[X++] Error "Voucher already in use as of date"

When seeing the error message
"Voucher CEU-000001 is already used as of date 31.12.2014."
(sequence number and date can vary) it is based on duplicate number sequences.

In my case it appeared when posting a sales invoice.
The number sequence for Sales order - free text invoice was set up with the same format as the number sequence for Sales order - invoice.

To fix the number sequences, I went to Accounts receivable\Setup\Accounts receivable parameters\Number sequences and changed the format of one of those two number sequences to make it unique:

So always set up transactional related number sequences with an unique format!