There must be somthing in the Excel sheet that Mendix does not like. Could it be that there are diacritic characters inside? I seem to recall there could be some issues in the latest version of Mendix with such characters. You might try cleaning those up before doing the import.
Thanks! I have tried already to take out some Characters. Now I tried also to work with the excel sheet out of the Learning Path (Build an App from an Excel Spreadsheet) and this also does no work.
Hey! Weekend is over and now it is working as it should.