Backups Fail: Server: Msg 3013

For some reason my backups (Transaction log and full backups) have started failing. When looking at the job histoy for the step that failed it reports:

*****(0.011 MB/sec). [SQLSTATE 01000] (Message 3014) Backup, CHECKALLOC, bulk copy, SELECT INTO, and file manipulation (such as CREATE FILE) operations on a database must be serialized. Reissue the statement after the current backup, CHECKALLOC, or file manipulation operation is completed. [SQLSTATE 42000] (Error 3023) Backup or restore operation terminating abnormally. [SQLSTATE 42000] (Error 3013). NOTE: The step was retried the requested number of times (3) without succeeding. The step failed.*****

I have tried running the SQL for the step manually and I get:

Backup, CHECKALLOC, bulk copy, SELECT INTO, and file manipulation (such as CREATE FILE) operations on a database must be serialized. Reissue the statement after the current backup, CHECKALLOC, or file manipulation operation is completed.
Server: Msg 3013, Level 16, State 1, Line 36
Backup or restore operation terminating abnormally.

The step is only determining what day of the week it is and putting it in to a '@bkupday' variable then: "BACKUP LOG [model] TO @bkupday WITH NOINIT , NOUNLOAD , NAME = N'Model Log Backup', NOSKIP , STATS = 10, DESCRIPTION = N'Model log backup', NOFORMAT" for each database and log.

This had been working fine, but for some reason has now started playing up. (trying to workout wether this ties in with when I started merge replication of one of the DBs)

Any one got any ideas as to what might be causing this as it's happened for a couple of nights now?

Cheerz.