If you synced sales orders to QuickBooks, there was an update on Friday that caused this error:

Error 3140 : While trying to Add a(n) Sales order with order number of 9726, QB responded There is an invalid reference to QuickBooks Customer SalesTaxCode "" in the SalesOrder.

Our development team is uploading a change this morning to address the issue. Please resync after 12 pm EST:

https://help.syncwithconnex.com/hc/en-us/articles/115003851203-How-do-I-sync-sales-manually-to-QuickBooks-