Adding to this thread for similar problems I had. This feature needs product enhancement, better documentation, and better training of support personnel.
I did a large file upload for event registrants (like the original poster). It displayed the banner saying that the file was accepted and that an email would be sent when complete. Then nothing. No registrants added, no email, no indication of how long to wait before deciding something went wrong. I contacted support and spoke to a rep on the phone. Then I went back and did more explicit tests. Here are the results and indications of problem areas:
1) If something goes wrong with the upload, there is no indication to the admin. An email should be sent stating that the upload failed (ideally with the first bad record indicated to help troubleshoot the problem).
3) My support person stated explicitly, and repeated when asked to confirm, that ONLY the 26 characters of the English alphabet are permitted. No spaces, no hyphens, no periods. On testing, all of these worked fine and were accepted. Give support a better list of permitted/forbidden characters.
4) What fails (and this is properly documented, so it's not a bug) are non-English characters such as accented vowels, Spanish n with a tilde, etc. This is an area that should be enhanced in future product releases, as American English names are not the only ones attending webinars. If the name fields on a registration page can accept the characters when typed in, they should also work for bulk upload.