I experienced the same behaviour but randomly. Never had a chance to try to figure it out but it would also occur in the sample database as well where I have never added any departments or masks before. It was just easier at the time to create the masks with a text editor.
I was always able to edit or delete any masks that had already been created.

While we are mentioning Departmental masks - Is there a formula that allows us to print the mask's description on the financial sheet.

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Don Wise
Westboro