Azure Portal: Bad Request - Request Too Long
You also may want to check b2clogin.com described here. As per Microsoft:
- Cookies are no longer shared with the other Microsoft services.
The error HTTP 400: Size of header request is too long generally happens because there's too many cookies or cookies that are too big.
Azure AD B2C's login goes through login.microsoftonline.com, as does almost every Microsoft service (O365, Azure, etc). So if you've got several accounts that you've signed in to across these services, you're accumulating cookies that will cause this problem.
This is bound to happen much more frequently to developers than end users as developers are logging in to the Azure portal with their corporate account, maybe also with a B2C admin account and then testing out their B2C-powered app with multiple logins.
In the long term, the answer will be is to allow Azure AD B2C customers to specify their own custom domain. This gives the application's B2C cookies isolation from everything else in login.microsoftonline.com. As of 2019-06-23, this feature is still under development. You can support this feature and keep track of its progress by voting for it in the Azure AD B2C feedback forum: Customer-owned domains
However, in the interim as workarounds, there are two things you can explore:
Clear your cookies. This will definitely work every time, it's just cumbersome, especially if presented to your end users.
Limit the amount of claims you include in your token. The more attributes you include in your policy, you'll end up with longer http requests which give you less margin for cookies from other Microsoft properties
Note: This is the same question as: http 400: size of header request is too long when signing in user using Multifactor authentication
2018-11 UPDATE:
Azure AD B2C allows you to use b2clogin.com instead of login.microsoftonline.com which will reduce your substantially reduce your exposure to this issue as you'll no longer share cookies with other Microsoft services.
2022-05 UPDATE:
Customer-owned domains is now live, updated the answer accordingly using strikethrough. Also, fixed the feedback link.
If you are encountering "HTTP Error 400 Bad Request - Request Too Long" for your azure account, you may also want to check whether the URL has been updated by microsoft.
In my case, I wanted to check on my Azure subscriptions. I used to go to this URL: https://account.azure.com/Subscriptions
But very recently it started giving me the "Bad Request Headers Too long" issue. I checked the URL and found out that it this is now the correct place to access my subscriptions: https://account.windowsazure.com/Subscriptions