System Message 15002 when setting up Site

Client is on SL 2018 CU7. When setting up a new site in Inventory, they are receiving a System Message 15002 "The entered sub account format does not match the format definition".

I have looked at DMG_AcctSub_Sub, the procedure used in the PV, and it returns the rows I would expect. The account and sub are set up in Account/Subacct Maintenance. The subaccount is a simple three character setup. I have checked for errant spaces. The screen is not customized.

This is the first site they've added since upgrading from SL2011FP1.

Thanks!