At the bottom of the filter setup form is the error handling setting:
This setting will capture the "standard error" output if your filter uses that for error logging. It will also generate a message in the event log if this setting is on.
We are aware that some programs have their own error logging and create a file that RPM may not know about. Of course, this is not a problem. You can create all the files you like but you'll be responsible for cleanup.
Our recommendation is that you turn on error logging for a time when you set up your filter action until you know that the setup works reliably. Only you would know how long that takes.
Note that you are responsible for removing the error logs, which is why some sites do not leave that setting enabled.
Enable the error log and study them; remember that these are errors in your program or script, not errors caused by RPM
You should also review the message log for messages related to your job. I have personally witnessed command-line errors or other runtime complaints this way which may not surface otherwise. We have instructions for executing your filter command interactively.