Log shipping failure ( sqlmaint.exe failed )

Log shipping was set up between 2 SQL Serve 2K EE with SP4 on windows 2K3 servers...We utilized the maintenance plan to setup the log shipping.
Recently nothing changed the copy job started failing sporadically, when checked the job history shows "sqlmaint.exe failed. [SQLSTATE 42000] (Error 22029). The step failed."
then I turned on the switch -Rpt to a text file... no matter the copy job succeed or fail, it only write a line to the txt file
" Microsoft (R) SQLMaint Utility (Unicode), Version Logged on to SQL Server 'SQL Server Name as 'OPRSVC_SQL_SMS2K3' (trusted) "
Really can not figure out the problem, sometimes the copy job succeed, sometimes it failed. Really don't know why...
Gurus here experienced such situation before, and also could not find something useful from KB or other posting. thanks a lot for you info in advance..