Hello to the forum!
Similar situation for me.
I am spending a lot of hours trying to resolve this issue. But, now, it is time to ask your help.
I have this VPS server with only one public/shared IP.

I have setup a Vhost with one only domain (registered with Registrar with DNS management correctly configured) running a small Wordpress network (The installation went Ok, but I have experienced a lot of permission denied and forbidden and so all, when in wordpress admin). Anyway, until this afternoon I was able to install wordpress, setup a working mail server, sending and receiving email via roundcube webmail, at the price of security setting
;Check wheter script is within DOCUMENT_ROOT
check_vhost_docroot=false
in /usr/local/etc/suphp.conf. Thinking about that, in fact, I am still in "development mode", I revert to
TRUE the line above.
Result: 500 internal server error. Also in roundcube.
I have made all the actions suggested here in the forum and in the Wiki: fix permission thousands of times, re-upload manually wp-admin and wp-include folder in their proper location, renamed .htaccess (Leave it untouched as default wordpress network required), rebuild virtual hosts, increase in the php.ini memory limit to 256 Mb, disabling all plugins, nothing.
Looking inside apache error log, it says at the recent few lines:
[Tue Aug 04 21:08:27 2015] [error] [client 93.41.199.45] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:08:27 2015] [error] [client 93.41.199.45] Premature end of script headers: themes.php
[Tue Aug 04 21:08:28 2015] [error] [client 93.41.199.45] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:08:28 2015] [error] [client 93.41.199.45] Premature end of script headers: themes.php
[Tue Aug 04 21:11:32 2015] [error] [client 180.76.15.146] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:11:32 2015] [error] [client 180.76.15.146] Premature end of script headers: index.php
[Tue Aug 04 21:13:21 2015] [error] [client 180.76.15.153] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:13:21 2015] [error] [client 180.76.15.153] Premature end of script headers: index.php
[Tue Aug 04 21:18:26 2015] [error] [client 93.41.199.45] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:18:26 2015] [error] [client 93.41.199.45] Premature end of script headers: themes.php
[Tue Aug 04 21:32:51 2015] [error] [client 93.41.199.45] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:32:51 2015] [error] [client 93.41.199.45] Premature end of script headers: index.php
[Tue Aug 04 21:32:53 2015] [error] [client 93.41.199.45] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:32:53 2015] [error] [client 93.41.199.45] Premature end of script headers: index.php
[Tue Aug 04 21:52:28 2015] [error] [client 66.249.78.57] ParsingException in IniFile.cpp:198: Malformed line "[global]\r"
[Tue Aug 04 21:52:28 2015] [error] [client 66.249.78.57] Premature end of script headers: index.php
At this point, any suggestion is appreciated.
Thank you.
David