1. Yes, BackupExec agent can be the case. Both can conflict if different VSS sequesters are trying to use VSS and certainly if one of them has a locked snapshot or leftover snapshot somewhere... Never a good situation to be in.
2. What OS is this? Is the 2008 R2? Did it get the latest patches and VSS hotfixes? it needs to be up-to-date. VSS is pretty sensitive to that
3. You will need to reboot first now. Because you see the Non-retryable error on NTDS, the only way to put the writer back into a good state is by rebooting your server.
The most logical thing to do (imho) is reboot, make sure it is fully patched, reboot after the patches and try again. But I fear that the problem will be with the BackupExec Agent. So if you have the possibility to uninstall that one... After all, you don't put 2 AV solutions at the same box either
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