Thanks for posting the question.
It appears to me that the issue is probably not with the where clause; rather it's an issue with the event text (or alert message) that you are attempting to log when the threshold triggers. Under circumstances where you are using 'NOTEXIST', you are telling the threshold to alert when there is no data that matches the where clause. As there is no data, there's nothing that can be substituted into the event/alert text.
Perhaps you can check for any field substitutions in your text (noted with a @RECORD.FIELD syntax) and remove that - and then see if the threshold will start.