when Alfresco is configured for Kerberos SSO, HTTP header sizes sent by the client can get quite large. This requires configuration changes in Tomcat, which is easy enough to do:
increase the maxHttpHeaderSize in Tomcat
However, the integrated Jetty server used for Sharepoint uses a default of 8192 bytes, which is configured in the AMPs vti-context.xml, bean vtiServerConnector, property headerBufferSize. This should be set to 32768 by default, so that users who are members in lots of groups can access SPP (kerberos header size depends mainly on the number of groups a user is a member of).
In addition to increasing the default, it might be a good idea to make its size configurable.