Just making sure here, but I assume you have made the same change to the maxHttpHeaderSize value in the server.xml after Tomcat 7 was installed, cause the BO installer wont make that change for you.
@Patrickperrier has this documented in http://scn.sap.com/community/bi-platform/blog/2013/11/08/warning-patching-your-sap-bi-40-to-sap-bi-41-will-break-your-ad-sso
On another node - can't you (I really dont know if it works) just increase the header size to, say, 1024*1024 = 1048576 ?