This SharePoint Online tutorial explains how to solve the error,
While browsing SharePoint Online sites in Google chrome it gave the error as http error 400. the size of the request headers is too long. When I tried to open in Internet Explorer, the SharePoint sites were opening fine. The error looks like below:

http error 400. the size of the request headers is too long. SharePoint Online
To solve the issue you need to clear Google chrome browser cache.
In Chrome click on : -> More Tools -> Clear browsing data like below:

Then click on Clear Data to clear Google chrome cache.

Once you clear the Cache, you will be able to open your SharePoint Online site.
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I hope this will be helpful to resolve the error: http error 400. the size of the request headers is too long SharePoint.
I am Bijay a Microsoft MVP (8 times –Â My MVP Profile) in SharePoint and have more than 15 years of expertise in SharePoint Online Office 365, SharePoint subscription edition, and SharePoint 2019/2016/2013. Currently working in my own venture TSInfo Technologies a SharePoint development, consulting, and training company. I also run the popular SharePoint website EnjoySharePoint.com