Keep just one cell with data validation (value box) operatable in a protected excel sheet

In my Excel 365 worksheet: I want to lock my worksheet "Home" but just let a data validation value box (not form or active-x combobox), assigned to a cell ("I6"), operatable.
That means the users shall be able to select a value from the data validation box, but not be able to change the format and other things.

I have tried so manythings but didnt work. In general I have tried this way:

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But each time the worksheet gets protected, an I cannot select a value from the data validation value-box in cell "I6".

When I comment out the line starting with .Protect, the whole sheet gets unprotected. So I am sure that there is no other code part that is causing the issue.

To solve this issue, the cell "I6" must be unlocked, and the worksheet must be protected with settings that allow the use of the data validation dropdown.

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