To determine that this is not an "Excel" workbook issue - save the worksheet
as a .CSV file and import that. If you encounter the problem again, it
indicates a "data related" error. If the error isn't encountered
immediately, you'll usually see records imported into your target contact
folder so for this exercise - create a new folder and import into it. You'll
be able to see the number of contacts imported which will in turn tell you
very quickly, which record in your input data is causing you the
aggravation.

If it happens immediately with nothing imported, then the suspect is the
"header row" itself (or the first data row if you were able to <map the
fields>) with something invalid contained within one of the fields.

If none of the above works, please try downloading one of our programs and
trying it (link below). If the error occurs within the first 50 records, it
will either give you an error message or reject the record and highlight it
in the error log file. If our program fails using your data, send a message
to support and it will generally be fixed within 24 hrs once we receive a
sample of your data that causes the problem.

Karl