We use IIS with enabled Kerberos authentication.
This issue occur when the user is a member of many Active Directory user groups. When a user is a member of a large number of active directory groups the Kerberos authentication token for the user increases in size. The HTTP request that the user sends to the CQ server contains the Kerberos token in the WWW-Authenticate header, and the header size increases as the number of groups goes up. If the HTTP header or packet size increases past the limits configured in CQ, CQ reject the request and send this "Bad Request" as the response.
*WARN* [10.6.18.198  <parse>] servletengine Bad request. com.day.j2ee.servletengine.BadRequestException: header exceeds 8192 bytes
In CQ 5.4 it was possible to change header size in server.xml, but in CQ 5.5 CQSE is a bundle.
How to increase header size in CQ 5.5 ?
As Sham states in the linked message, you need to request for the FP NPR-2324.
But imho, you should look into how you group the access groups in AD.
And what is FP NPR-2324 ?
Open a Daycare ticket and ask for Featurepack with the name "NPR-2324".