The most likely cause for this issue is that the anonymous user assigned for use to browse the website is out of sync.

To resolve the issue, please do the following:

Login into Windows through Remote Desktop and go to IIS manager, right click on the site in IIS experiencing this issue and select click properties.
Left click Directory Security, click EDIT under Authentication and access control.

Enable anonymous access should be ticked.
Intergrated windows authentication should be ticked.

Update the password to be the same as the FTP password for this account in plesk.
The anonymous username should look like this:

IUSR_ftpusername

where ftpusername should be replaced with the actual ftp username used for this domain.
Click OK and the click apply.

If that does not work, change IUSR to the servername. So, if servername is AYKS1, it should look like AYKS1_username


If that doesn't work, there is a hot fix from SW-Soft.

http://download1.swsoft.com/Plesk/Autoupdate/pleskVERSION_updatefile.msp

Download and install the hotfix. The run the following command from the command prompt in Windows:

cd %plesk_bin%
websrvmng.exe --update-anon-passwords-all