Btrieve Error: File Access Denied (error=46) Quotcost_new_cost: onecost_insert  (Thread 0)  
Error: Btrieve Error: File Access Denied (error=46) quotcost_new_cost: onecost_insert (Thread 0) 
This error was presented sporadically when opening a new quote. Selecting OK would allow the user to continue, but all of the Additional Costs were missing. 
Solution:  
Remove all duplicate invoice or quote numbers to permanently remove the error.