It was so simple that an attempt had been made to post two purchase invoices at the same time, thereby creating too many characters in the invoice number.
Well You should talk with someone who has configured Your system or helped to set it up.
The payment file can be seen, when it is generated, and which accountants use to import to Bank, for payment processing.
But how this Payment File is created, there is no way for us to tell at this point. It could be standard - created from Data Exchange definitions, or it is some add-on or maybe localization.
In the previous error it is weird that it shows in red only like 9 digits, but error states it cannot be more than 20 characters.
You are telling, that this appears when You try to post the Purchase Order, but in the picture it is related to Payment File.
Are You generating and exporting a payment file at the moment of Posting Purchase Order? I would suggest to check how the timestamp is created in this file. Seems the problem is there.
At the moment we don't have information if this is standard, or an add-on. Could be that just by changing Your regional settings, this issue would be resolved - as the additional functionality was developed with different settings, and no one has checked other Regional settings.
I assume there are no problems to post such Purchase Orders, where other Payment Method is used?