Debug Assertion Failed! - Failed Export

Overview:
SQL Server 2000
Windows 2000 Advanced Server
Problem:
We have a bespoke Sales Order System using SQL (called Bison).
We raise quotes for customers, which get converted into Sales Orders then to Invoices.  We print the invoices and the status in Bison gets changed to "printed" we then run a routine Export Customer Transactions which dumps out a txt file of all invoices and changes the flag to "exported", we use the txt file to import into Sage Line 50.  
Our last export did not change the flag from "printed" and the txt file was created but missed off the last 40 invoices, perhaps because it had a Debug Assertion Error mid-way through.
What we saw:
Debug Assertion Failed!
Program: c:\bison\debug\MaintainAccounts.exe
File e:\projects\bison\source\Accounts\AccountsIO\H_SageLine50_ops.cpp
Line: 451
(press retry to debug the application)

On pressing retry we got the same error in lines 471,1456 and 518
Then the following error:
e:\projects\bison\source\Accounts\AccountsIO\H_AccountsMaintenance.cpp
Line:138

The person who wrote this application is no longer at the company! and I am only Admin experience with this.  I would appreciate any help to try and find out how to go about resolving this, and/or setting the invoices manually to "exported" to see if the next time I run the "export" i get the same error?
Many thanks in advance.