1.) Make sure that ANY local volume shadow copies you're taking, are turned off. We've had QUITE a bit of conflicting information from Symantec regarding running local VSC, but have found that when we talked to the techs, they've all advised we don't run VSC on the same server.
2.) If you absolutely, positively have to run VSC, (like, there's a group of users who love deleting things and making you restore them via VSC and they're holding your family hostage or something) you could try resetting the entire kit-and-caboodle by using "vssadmin" to delete all current present volume shadow copies, then delete them.
In addition to the above, have you presented/removed a disk, (such as from a SAN via fiberchannel, etc?) recently? Make sure it's no longer present in your system, and that you haven't inadvertently attempted to take a volume shadow copy snapshot of it.
Along the same side of the coin, you may have "stale" volume shadow copy snapshots present for disks that are no longer there. That could be exacerbating your issue. Suggest same above, to delete those, since I believe that vssadmin is the only way you can see some of them.
Don't quote me on this, but I think that you also have to expand the default size of the VSC from 300 to 500, however I'm not entirely sure. I do remember that it took me quite a bit of googling though and in the end, this made Symantec happy.
Sorry, ordinarily I'd be more clear and try to give some steps but I can't seem to find where I documented this, the last time it worked and the primary backup guy is out for the day, (only other guy who'd have them in his email.)