User is getting an error when creating a requisition via iProcurement - FND_FLEX_SERVER validation f

Our client has reported an issue when creating requisitions using iProcurement.

When they are at the checkout screen and click on "next", they get an error message as follows :

Account generation failed.

Charge account is invalid (Program error: an error has occured while looking up a flexfield segment combination. Please inform your support representative that: the FND_FLEX_SERVER validation function has encountered a null or negative combination ID () while trying to look up the segment values for this combination. This may indicate an invalid entry in the unique ID column of the combinations table)

If the user logs out of iProc, clears their cache and then logs back in, the error is not there and they can progress the requisition and checkout successfully.