Additionally, users will sometimes encounter errors due to cache issues with their browser. Cache is stored for increased loading performance, but too much can accumulate and bog down processes. I'm including an article that goes over this in more depth and provides steps for supported browsers: Delete or Disable Cache and Temporary Internet Files in Your Web Browser.
These resources should help to get you back on track and avoid errors when importing customers.
Please let me know if you need anything else. Have a wonderful day.