Type mismatch error for countif function

I made a short script to update a table with values, that are counted in another workbook. Unfortunately I get a type mismatch error for the Countif function. I suspect, that I define the selected range wrong. Could someone tell me, what I'm missing here? Here is the code I wrote so far:

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I tried something out to analyse the issue further. I copied the source table into the same workbook, where the macro is located and the Countif function works fine there. So it seems that the problem is with referencing the range in the other workbook.

Fortunately I got an advice to open the source workbook within the same instance and that solved the issue