RESOLUTION
This error means that web site sqladmin(default) responsible for mssql web admin is incorrectly configured in IIS. To fix the site, the following command needs to be launched in the command prompt on the server:"%plesk_bin%\websrvmng.exe" --reconfigure-sqladmin
"%plesk_bin%\defpackagemng.exe" --fix --type=sqladmin.mssql