Run-time error '-2147417848 (80010108)': Automation error

Run-time error '-2147417848 (80010108)':
Automation error
The object invoked has disconnected from its clients.

Error occurs under the following conditions:
Run macro to call a form. See code below. Error occurs on last line.
On all computers with a hidden Excel file name SUMacLib.xlsm.
Only when all Excel files are closed (except the hidden SUMacLib.xlsm file is open).
Only after Excel has been used for more than approx. 30 minutes and Excel files have been opened and closed many times.

Solutions attempted:
Unhide the SUMacLib.xlsm file - No error
or
Keep open any Excel file - No error
or
Modify the form on the SUMacLib.xlsm file - Error
or
Create new form in the same file - Error

Proposed solution:
To avoid the error when the form is called and when all Excel files are closed and keep the SUMacLib.xlsm file hidden...

Macro:

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Additional Information:
TemplateDia.Show contains a window that shows a list located in the SUMacLib.xslm file. An item on the list is to be selected, then an "OK" button is clicked to run additional macros.