For those people who interact with many sites across the *.Microsoft.com domain this may be an issue. I was looking for an update on Office Add-Ins and I came across this issue.
If you see a 400 Bad Request Size of a header exceeds the limit error, for no good apparent reason, that cause is likely cookies.
The following image was taken from just trying to access the Microsoft 365 Blog site. As you can see there are a LOT of cookies picked up from all over the MS domain.
The simple solution, clear your cookies. This process will vary per browser but once that is done, we are in business again.
Microsoft is not the only company with this issue as I wrote about before 🙂