Event code: 0x01350003+0x0009006F

I have Acronis backup and receover 11 running on windowd server 2008 and
I have received the following error when i ran an diffrential backup:

TOL: Command 'Running backup plan' has failed.
Additional info:
--------------------
Error code: 3
Module: 309
LineInfo: 4a8728dc8a1c94f8
Fields: $module : C:\Program Files\Acronis\BackupAndRecovery\mms.exe
Message: TOL: Command 'Running backup plan' has failed.
--------------------
Error code: 111
Module: 9
LineInfo: d6e6b1a4069ca4f9
Fields:
Message: Failed to run the task: it is already waiting.
--------------------
Please advice,
Regards
Waleed Gouda