I found this in the Knowledge Base, yes I know, a cool place to research error messages. Not the same line number but what the heck, give it a try.

Error: "41 AP_PeriodEndProcessing_ui.pvc 785" in Period End Processing

Cause

There is a record with an invalid date format.

Resolution


Open Accounts Payable, Period End, Period End Processing
Select Proceed to re-create the error.
Select Info to access the error information window.
Select Debug to access a Providex debug window.
In the Debug window, at the -> prompt, enter each Debug command below, followed by Enter, and note the result before proceeding with the next Debug command:
? OpenInv.APDivisionNo$
? OpenInv.VendorNo$
? OpenInv.InvoiceNo$
? OpenInv.InvoiceDate$
Note: The last value should be the Invoice Date value, which is likely incorrect or corrupted. Normal date values are in the format of YYYYMMDD.
To exit the Providex Debug window, enter the following command and Enter key: BYE
Open Library Master, Utilities, Data File Display and Maintenance.
Locate the AP_OpenInvoice.m4t data file for the company and select Key Scan (magnifying glass) icon button to locate the specific vendor invoice record. The Debug results obtained should provide key values to find the correct invoice record.
Select the invoice record.
Examine the following fields for date values that are not in the normal format of YYYYMMDD (e.g. "June 2, 2009" should be "20090602" and not "90300602"). Incorrect or corrupted date values will have to be edited to the correct value if they are not in the correct format:
InvoiceDate$
InvoiceDueDate$
InvoiceDiscountDate$
CheckDate$
Select Edit, access and edit each field and edit the date to be in the correct format.
Note: To determine the correct date(s), it may be necessary to access physical A/P Invoice Register printout records or the Accounts Payable, Main, Invoice History Inquiry
The same date value check must be performed on the corresponding invoice record in the AP_TransactionPaymentHistory.m4t data file for the company.
The fields to check for corrupted dates to edit are:
TransactionDate$
CheckDate$
After correcting the dates to the proper YYYYMMDD format, close out of Data File Display and Maintenance.
Attempt again to perform Period End Processing in Accounts Payable.
Note: If the error occurs again, follow again the steps to re-create the error and enter the debug commands to obtain invoice record information. It is possible that more than one invoice record may be incorrect or corrupted. If many invoice records are suspected of having this problem, it may be necessary to create a custom Crystal Report, pulling data from the AP_OpenInvoice.m4t data file to sort by and check the Invoice Date fields.