Found this on Sage's old site.
Subject:
Error 29 ""Error in Definition of Screen item"" or ""Invalid Mnemonic or position specification"" occurs after resizing a window.
Note: The error may be workstation specific.
Possible Resolutions:
A tab may have been resized in Customizer so that it is larger than the main form. Perform either of the following:
Resize the tab so that it is smaller than the form.
Open the task and press CTRL-F9.
Select the customized form and click OK.
If a specific tab appears, close it to display the main form containing the tabs.
Right-click anywhere in the form to display the Folder Definition window.
Adjust the width and height as necessary, and click OK.
Delete the customized form.
Open the task and press CTRL-F9.
Select the customized form and click OK.
In the Customizer tool bar, click the Delete button (red X).
Click Yes at the confirmation prompt.
Another customization may be causing the error.
Open the task, but do not click the tab that caused the error.
Right-click anywhere in the window.
In the context menu, point to Form Settings (or Panel Settings), and select Reset Form.
Select the following check boxes and click OK:
Reset Window Size and Position
Reset All Grids
If the error still occurs, delete the customization record from SY_Personalize.m4t.
Note: Use caution when working with Data File Display and Maintenance. Always make a backup first. If necessary, seek the assistance of a qualified Sage Software business partner or Sage Software Customer Support analyst .
In Windows Explorer, go to ..\Mas90\Mas_System.
Make a backup copy of Sy_Personalize.m4t.
In Sage MAS 90, expand Modules, Library Master, and Utilities. Double-click Data File Display and Maintenance (DFDM).
Browse to and select ..\Mas90\Mas_System\Sy_Personalize.m4t.
Select the record to be deleted.
Use the Librarypath$ and Workstationname$ columns to identify the record. For example, if the error occurs when opening G/L Account Maintenance on workstation Cmp0124, search for a record with the following values:
Librarypath$ = Mas90\Gl\Gl_Account.m4l""
Workstationname$ = Cmp0124
Click the Delete button to delete the appropriate record.
If the error occurs in several tasks, rename Sy_Personalize.m4t. The system recreates this file automatically.