Where did you sign in from?

www.bing.com/businessportal ?

Here bingads.microsoft.com

Hmm...

Try logging in through www.bing.com/businessportal (the login will be at the top right corner). Then, once you get to the portal it'll say "Business" and "Ads" at the top left. Click Ads.

Let me know if that works.

I found this online:

"With the transition to logging in with a Microsoft account, it should be noted that there are two different sign in pages: a Bing Ads sign in page (for customers who have not yet migrated to Microsoft account) and a Microsoft account sign in page (for customers who have migrated to using Microsoft account to login to Bing Ads).

Once you migrate your Bing Ads account to Microsoft account authentication, information is added to a browser cookie to ensure that the next time you go to www.bingads.com, you will be automatically redirected to the Microsoft account sign in page."

I got that once also.
At first I thought it was a phishing attack...
But I really wanted to log in so I used my MSN login and it was OK....
just make sure you check out the domain you are logging into... see if it has the https prefix