I have 3 PCs on a home network that are being backed up by the software that came with a HP Mediasmart Windows Home Server. The day after installing NIS 2009 on one PC, my backups started failing (80% of them) on just that machine. The other PCs backed up fine, and the problem appeared for the first time right after NIS 2009 was installed.
I went into the NIS permissions settings and granted all internet access to each of the WHS modules, but error persisted. The specific error message was contained in the backup detail log-it stated there was an UnauthorizedAccessException and access is denied. The backup will fail with this message about 15 seconds after starting. It would fail on manual backups and scheduled backups. I added the home server to the trusted network but still error. I even tried disabling NIS and running the manual backup - still error continues. Contacted Norton support, they could find no additional settings possible. I reinstalled the Windows Home Server software - no fix. The solution was apparently a .net framework update. When I ran Microsoft Update, there was a non critical microsoft .net framework 3.0 service pack 2 waiting, and the description did mention some compatibility and connectivity issues. I installed the (large) update and the manual backup worked after reboot.
Can't say for sure that it was NIS 2009 that surfaced the issue, but the timing was a big coincidence if it wasn't the cause.