We have a site collection, that we recently added an AD security group to that has been giving us random access denied errors. The thing is the permissions are set right, and it only gives us errors on one of the two front end web servers. The other allows access. With no changes made, the second node suddenly started allowing access again. The first time this happened, a reboot fo the server allowed access. I drain stopped the effect frontend web server today and was troubleshooting, when it magically started authenticating again on that node. At this time, everything is working correctly again, but this has happened two days in a row and am being pushed for a root clause. It only seem to effect the one site collection, as when I was testing all othe site collections, included the root authenticated with no problems. We have only recieved reports on the one site collection, and we are about ready to delete it and rebuild it, but was looking for a better answer.
The two front end web servers are members of an NLB cluster. The farm is a 3 server farm including SQL server, with SharePoint 2010 Enterprise on Server 2008 R2 servers and SQL 2008 R2 database cluster. The system is Patched up to the December CU.
Should add that the access denied has been seen on both web servers, but not at the same time yet.
Thanks
G