I was getting below error while importing solutoin in my trial CRM 2016 Online Org. I tried with other browser and it was the same case. Infact I created a new trial and tried there but no success.
Without losing hope, tried the same in one of my VM (Windows Server 2012 Standard). It got successfully imported.
So went back to my workstation and cleared browser cache there.
And the Import was Successful.
Hope it helps..