Not sure if this will help with your specific issues, but I was experiencing something similar before I tried these steps and I was then able to successfully import my CSV list:
Remove special characters in both first, and last name fields. I'm using Gsheets for editing the data and I first selected both columns, then "Find-and-Replace" from the menu to remove special characters like quotation marks, periods, hyphens, underscores, and spaces.
Make sure the CSV filename has NO special characters. "nameofyourfile.csv" VS "name-of your_file.csv". As you can see the second example here has a hyphen, space, underscore, and a period in the middle of the filename.
This worked for me and really I think it was the filename (STEP 02 here) that did the trick. Hope this helps!