I are having the same issue.
I'm using v10.0 SP5 and 5 remote clients. Two of those remotes are SQL 2005. The non-SQL servers have no problem at all. As for the SQL Servers, we can't back up anything on the server except for the VolumeCopy without getting the error. I want to back up the SQL Backups and a few other non-sql directories.
After further testing, I found that backups can run after turning off the SQL service. This proves that it's SQL that is directly affecting the backups.
This appeared to start happening after applying SP1 for SQL2005. Before that, backups were fine.
I've come across a few recommended solutions.
One, to rename the remote client's SQL DLL to .OLD or something so it doesn't load.
Two. Re-install the remote client.
Three. A registery change.
None of these worked. Last resort is to upgrade to 10.D or 11.D. I will be doing that soon, and report back.
Additional Information: I have two licenses for BE Windows Server. Both are running 10.0 SP5. One is a stand-alone and one with remote clients. The problem surfaced with the Remote Clients. However, I am doing the testing with what was the stand-alone by installing remote clients. This way I am not affecting my main backup server.