I'm also having an issue with the webmail after a fresh install.. im getting the 502 Bad Gateway error... I tried the previous steps you suggested to fix the issue..
B. OS: CentOS release 6.6 (Final) i686
C. Apps:
1. MySQL: mysql55-5.5.41-2.ius.el6.i686
2. PHP: php53u-5.3.29-1.ius.el6.i686
3. Httpd: --uninstalled--
4. Lighttpd: --uninstalled--
5. Hiawatha: hiawatha-9.11.0-f.1.mr.el6.i386
6. Nginx: nginx-1.7.9-1.el6.ngx.i386
7. Cache: --uninstalled--
8. Dns: nsd-4.1.0-3.mr.el6.i386
9. Qmail: qmail-toaster-1.03-1.3.45.mr.el6.i386
- with: courier-imap-toaster-4.1.2-1.3.18.mr.el6.i386
D. Php-type (for Httpd/proxy): php-fpm_worker
E. Memory:
total used free shared buffers cached
Mem: 5893 574 5318 27 41 346
-/+ buffers/cache: 186 5706
Swap: 6031 0 6031