Horde error 502 - Web server received an invalid response while acting as a gateway or proxy server.

If you are using a Windows 2008 server, running Plesk 8.6, you might encounter this error on Horde:

502 - Web server received an invalid response while acting as a gateway or proxy server.There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.

Fixes:

1. You need to see the complete details of what is happening in your IMP server. Browse locally or from the server to http://localhost:8425/.

2. Check if CGI Role Service is installed in IIS: Server Manager > Roles > Web Server > Role Services

3. Uncomment the following files on the PHP configuration file from %plesk_vhosts%\webmail\horde\php.ini. - php_fileinfo.dll, php_mysql.dll and bphp_mysqli.dll

4. Do no forget to set cgi.force_redirect = 0 in %plesk_vhosts%\webmail\horde\php.ini.