Mailchimp log in problems

[#1]

Since I upgraded to fibre I am struggling to log in to my Mailchimp account. All other sites are working fine. Any ideas?

[#2]

I'm assuming you're being presented with an error advising you of trying to log in with an incorrect username or password?
Obviously bearing in mind this began when you changed to fibre I'm assuming you've still tested this in another browser or have tried that site on another PC or another internet connection?