Seems like permission issue. Can you check cat /usr/local/apache/logs/error_log and see if it show you something. Also look in the link below:
http://wiki.centos-webpanel.com/500-internal-server-error
Hello! This is my error_log
[Fri Mar 20 04:26:25 2015] [crit] [client 37.147.209.9] configuration error: couldn't perform authentication. AuthType not set!: /
[Fri Mar 20 04:26:30 2015] [crit] [client 37.147.209.9] configuration error: couldn't perform authentication. AuthType not set!: /
[Fri Mar 20 04:26:33 2015] [error] [client 37.147.209.9] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.
[Fri Mar 20 04:26:34 2015] [error] [client 187.94.99.195] SoftException in Application.cpp:249: File "/home/admin/sadmeb.ru/index.php" is writeable by group, referer:
http://www.sadmeb.ru/promo/otbel.html[Fri Mar 20 04:26:34 2015] [error] [client 187.94.99.195] Premature end of script headers: index.php, referer:
http://www.sadmeb.ru/promo/otbel.html[Fri Mar 20 04:26:40 2015] [error] [client 95.167.189.108] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.
[Fri Mar 20 04:26:45 2015] [error] [client 66.249.67.17] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.
[Fri Mar 20 04:26:45 2015] [error] [client 95.167.189.108] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.
[Fri Mar 20 04:27:05 2015] [error] [client 141.8.147.2] SoftException in Application.cpp:249: File "/home/admin/sadmeb.ru/index.php" is writeable by group
[Fri Mar 20 04:27:05 2015] [error] [client 141.8.147.2] Premature end of script headers: index.php
[Fri Mar 20 04:27:05 2015] [error] [client 217.118.79.45] SoftException in Application.cpp:249: File "/home/admin/\xd0\xb3\xd0\xb5\xd0\xbb\xd0\xb5\xd0\xbd\xd0\xb4\xd0\xb6\xd0\xb8\xd0\xba-\xd0\xba\xd0\xb2\xd0\xb0\xd1\x80\xd1\x82\xd0\xb8\xd1\x80\xd1\x8b.\xd1\x80\xd1\x84/index.php" is writeable by group, referer:
https://www.google.ru/[Fri Mar 20 04:27:05 2015] [error] [client 217.118.79.45] Premature end of script headers: index.php, referer:
https://www.google.ru/[Fri Mar 20 04:27:06 2015] [error] [client 217.118.79.45] File does not exist: /home/admin/\xd0\xb3\xd0\xb5\xd0\xbb\xd0\xb5\xd0\xbd\xd0\xb4\xd0\xb6\xd0\xb8\xd0\xba-\xd0\xba\xd0\xb2\xd0\xb0\xd1\x80\xd1\x82\xd0\xb8\xd1\x80\xd1\x8b.\xd1\x80\xd1\x84/favicon.ico
[Fri Mar 20 04:27:07 2015] [crit] [client 141.8.147.5] configuration error: couldn't perform authentication. AuthType not set!: /dreves_grib.php
[Fri Mar 20 04:27:14 2015] [crit] [client 37.147.209.9] configuration error: couldn't perform authentication. AuthType not set!: /
[Fri Mar 20 04:27:21 2015] [error] [client 95.167.189.108] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.
[Fri Mar 20 04:27:22 2015] [error] [client 66.249.67.9] Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace.