The text below is directly borrowed from this post. I see it hasn't been resolved.
Hi, the list import wizard gets stuck in the second step. I’ve tried a couple times and I get the “spinning wheel of death” (it never loads). Anybody facing the same problem and any solution to this?
Things I have tried and have failed to generate results
Here is a solution that works, but is unacceptable for my needs
Thanks in advance
Things like this have happened to me as well, but I've almost never received good guidance or fixes from Marketo Support.
What does seem to be a common thread are non Western special characters that are in the cell OR already in the database that seem to cause a problem on import. One way to look is to pull down the list with use Email Address so you can examine the fields in Marketo without having the import break.
UTF8 is your friend.
You mention creating a brand new .csv with a different file name. Does this mean you saved the current version with a new name or have you copied the contents and pasted it in a new blank file and saved it again? The later could be worth a try.
Also, if it was urgent, you could try halving the data and uploading each half separately. Whichever half doesn't work you know it has the bad data. You could do this a few more times to narrow it down. This isn't at all a proper solution and if your list is huge it would take a long time.
When I can't figure this out, I send a support ticket. Eventually some engineer figures out the problem.