Hi Ulrich, the example file does work. For my test I had eliminated the last 2 columns - custom_usermeta 1 & 2. Here is what I've tried so far.
1. I opened your example file and simply overrode Column A, B with the names of two people. In Column C I put a password. First Name and Last Name I overrode with the people's names and I left the last two columns Custom user name as test just like you have it. I saved the file as import test and then imported it. What got imported was your example file with John and Jane Doe instead of the names of the 2 people I had.
I don't use spreadsheets so I had hoped to simply use your example file and copy my columns over. However is there something in the example file that would keep your original data even though I put in different data in the first 5 columns? I didn't touch the last 2, just left them alone.