Dear Andy Pope, it works to my surprise and relief! The reason is not CountA though, since the values for these particular dropdownlists are numbers and 'count' works too.
But thanks to your answer I now discover that the '$'-signes are mandatory. I did not use them because I have several dropdowns and I copied their range-formula from one ro to the next. Now I did the same, but added the $-signs afterwords and it is working like a charm. Thank you very much for your quick answer.

Also for some reason sometimes the range is converted to the same formula but between quotes!