It could be the problem:
Please check this:
http://www.snb-vba.eu/VBA_Fill_combobox_listbox_en.html
In the article:
"If you use 'rowsource' you make a direct link to a certain range in a worksheet.
Combined with the combobox it can cause Excel to blackout, because any change in the combobox will be transferred directly to the 'source'.
Excel has proven not to be able to do this correctly.
Besides: the changing of the source at every change in the combobox slows down your code.
You should reduce the reading/writing from/to a workbook in your code as much as possible. "
so replace with this:
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