I found out what the problem was. However, I'm not sure about the solution yet.
I've linked a VBA TextBox to the cell with the Exchange Rate, so whenever I change a value in the cell or in the textbox, it updates the other one. I did this because I want to call a VBA method when the textbox loses focus to refresh a pivot table.
I'm not sure how to trigger a VBA method when I step out of an Excel cell. If that is possible, it already solves my problem, because then I can get rid of the VBA textbox.
Back to the textbox. It is linked to the exchange rate cell, but when the designer fills the exchange rate cell, the linked textbox is not updated, so it still shows &=$ExchangeRate.
To workaround this I made a method Workbook_Open to fill the textbox based on the exchange rate field:
Sheets("Quote").txtExchangeRate.Value = Sheets("Quote").Range("C15").Value
This is where it goes wrong. Even though the value from C15 is e.g. 1,27 (comma), the textbox is updated with 1.27 (dot). And because the cell and the textbox are linked, the cell is also updated to 1.27 (dot)!
So, I see a few possible solutions:
1. Get rid of the textbox, but then I need to trigger a VBA method when I leave an Excel cell (don't know how to do that yet)
2. Get the designer to fill in the textbox with the same value as the cell. Now, even though they are linked, the textbox is not updated.
3. Change the Workbook_Open method so that the correct value is copied from the cell to the textbox.
Any ideas? :) Personally I prefer option 1.