Custom Alert not triggered

I’m currently testing the Custom Alerts plugin on v4.8.0. I created a new alert like:

Period → day
Send report to → Checkbox checked
Alert Condition → This applies to report: Visits Summary
Alert me when → Visits grater than 1

The alert was never triggered and therefore no mail was sent.
I have not found any error in the log files.
What could be the problem here or where could I look for the error?