Error number [2:???] means the application plugin failed and returned specific error code ???. This can contain a lot of different numbers. Some frequently displaying codes are translated to "human readable" format.
Error number [3:0] means the task execution (start of task by application plugin) failed due to an error in application plugin.
Error number [4:0] means the task cannot be passed to the application plugin (to the antivirus client in this case) because it is not available. This situation can be occurred by broken installation (less probability) or bccavsvc service is not running or is not communicating properly with aswbcc service.
Error number [5:-1] says that task was passed to application plugin correctly but result was not returned in defined time after task has been finished.
Error number [6:0] says that application plugin doesn't know anything about task.
All these errors except [2:???] one are quite obscure and should not display in normal usage. If it happen it is recommended to restart station and/or reinstall antivirus.