On the error window:

1. Click Info, Debug.
2. Enter the following commands and note their returned values:
   1. ? WOS$(1,7)
   2. ? WOR$
   3. ? WO1$
3. Open Library Master, Utilities, Data File Display and Maintenance. 
4. Browse to ...MAS90\MAS_xxx\WOxxx\WORxxx (xxx = Company Code).
5. Click the Key Scan button, locate the records returned from step 2.
6. Select record and click Delete.
7. Open Library Master, Utilities, Analyze and Relink Files. 
8. Select Work Order from the Module drop down, enter Company Code.
9. Select WOS, click Proceed.
10. Open Library Master, Utilities, Rebuild Sort Files.
11. Select Work Order from Module drop down.
12. Enter Company Code, click Proceed.
13. Click Yes to the message, "Would you like to recalculate Open Work Order Quantities in the Inventory Masterfile also?"