This error means that the web site webmail (default) responsible for webmail is misconfigured in IIS. To return it to the working state you need to apply Plesk utility that will reconfigure webmail on the server. Do the following:
1. Login to your Windows server as the administrator
2. Start --> Run
3. Put in the following commands one after the other to fix the issue:
"%plesk_bin%\websrvmng.exe" --reconfigure-webmail
"%plesk_bin%\defpackagemng.exe" --fix --type=webmail