From Plesk 12.0.18 Update #71 onward, website configuration saving location have changed from web.config file inside website doc_root to applicationHost.config file in C:\Windows\System32\inetsrv\config. So it is recommended to make any change in MIME Types, Error Document and Default Document through Plesk Control Panel > Websites & Domains > Virtual Directories only. Also many IIS settings are now mapped through Plesk Control Panel > Websites & Domains > IIS Settings in this new versions of Plesk Control Panel and should be managed from there.
Changing the above mention properties in web.config can conflict with applicationHost.config and generate IIS error.
Article ID: 805
Last updated: 09 Dec, 2015