The following message can be seen in the Agent log when File Event runs even though the event completes successfully:
U02000123 Error in function checkFileStable(): the system cannot find the file specified
Release : 12.3
Component : Automation Engine
This is a designed behavior which states that the file does not exist.
Once it does exist, the file event should be satisfied and the Process tab is executed, the event will complete fine when the file is found.
By default, the Agent will check every 60 seconds when using the option FILE_STABLE.
The frequency can be changed with EVENT_CHECKINTERVAL in the UC_HOSTCHAR_DEFAULT.