Hi Adam,
We would need a 'copy support' collected from the 6720 during the problem state, then another 'copy support' after you reboot it to see why that IP is being blocked.
In general, if the VDX should be on 4.1.3d which contains all the latest fixes. If it's not on 4.1.3d firmware yet, I highly recommend upgrading to it, then verify if you still have the issue.
If you still do, then please collect the data requested from above during the problem and after.