This article shows where you can access www server logs in Plesk.
First open the "Websites and domains" page. In the domain's options you can find the "Logs" selection which takes you to the log viewer.
The log viewer allows you to filter log lines based on for example IP-address, HTTP status code or the log body text. The top right corner drop-down menu allows you to select which logs to show. The screenshot above has selected to only show error messages.