When performing an Exchange Upgrade to 2016, I performed a client access switchover and encountered a single user who was receiving a number of errors in Outlook. When troubleshooting we found that autodiscover was not working for this user (it worked fine for everyone else) and when navigating to the Autodiscover URL, they received a “HTTP 400 Bad Request” page.
After a bit more digging I found the below kb:
This arctile led us to the solution of the issue as it turned out the user was a member of nearly 100 AD Groups and the token used in the proxy request for autodiscover was simply too large.
We confirmed the issue by removing a number of groups for the user and low and behold they were then able to open Outlook successfully. We then implemented the registry fix on the 2010 Client Access Servers as per the article and added the user back to his groups. Happy to say everything is working as expected.
As this is a proxy issue, the below workloads can be affected by this issue:
Hope this post helps you in some way.