I resolved this issue in my situation as follows:
Checked the log errors on the server and found the following:
[Mon Aug 11 17:29:56 2014] [error] [client 198.241.168.60] ModSecurity: Access denied with code 406 (phase 2). Operator EQ matched 0 at REQUEST_HEADERS. [file "/usr/local/apache/conf/modsec-imh/19_bad_bots.conf"] [line "13"] [id "13009"] [rev "1"] [msg "Request Missing a User Agent Header"] [severity "NOTICE"] [tag "OWASP_CRS/PROTOCOL_VIOLATION/MISSING_HEADER_UA"] [hostname "www.my_domain.com"] [uri "/plugins/system/success.php"] [unique_id "U@k11BfrwscAADr1OQcAAAAj"]
The hosting provider recently enabled a new Mod Security rule which requires a User Agent Header in my file. My file, success.php, is using an http request type without declaring a user agent type. Therefore, access was denied and timing out. I had support disable the Mod Security rule until I can revise the php file to include a user agent type. Hope this helps others.
See Document Conformance at http://www.w3.org/TR/xhtml1/normative.html