List Import Trouble: "The list is importing another list."
I'm having trouble uploading lists. When I go through and hit "Import", it displays a red error message: TThe list is importing another list. Wait for it to finish and try again.

I've been using Marketo for a year and a half and have never seen this issue before last week. However, it's happened on three separate times that I've tried it and for different lists in different Marketo campaigns. I'm not uploading a different list at the same time, so I'm not sure why it's saying this.
Has anyone else run into this issue and how do you resolve it?