Arcserver 2000 errors when i try to back

I recive the fllowing error when i try to back sql server 6.5 with sp 3
E8604 Failed To start backup. (DBNAME=Quality,
> EC=Backup Agent error – (73) “SQL Server error. Please check the log file

Cannot Dump Tran: Insufficient free log data pages
> (threshhold might be exceeded.) Please refer DBagent trouble shooting

can someone help me