Sage 50US 2023 .NET SDK, calling PurchaseInvoice.Save() and received "Can't start Btrieve Transaction."

Stack Trace:
Sage.Peachtree.API.Exceptions.PeachtreeException: Can't start Btrieve Transaction.
Sage.Peachtree.API.Entity.SaveInternal()
Sage.Peachtree.API.Entity.SaveTemplateMethod()
Sage.Peachtree.API.PurchaseInvoice.Save()

This does not happen to all of our clients, but quite consistent on one of them.  Tried to look for Btrieve error in "zen.log" but there is nothing.  What could be the cause and what should we look for?  Too many concurrent users?  On what tables?  I don't find any mention to transactions in .NET API to avoid this.

Regards.