I set up this new site because my original site ended up having the same issue I'm about to describe.
After activating the WPMU Domain Mapping plugin I am unable to access my Network settings page. There are no entries in the site error logs but the Apache2 error log shows the following:
[Sun Feb 08 21:06:02.048103 2015] [mpm_prefork:notice] [pid 7526] AH00169: caught SIGTERM, shutting down
[Sun Feb 08 21:06:03.203895 2015] [mpm_prefork:notice] [pid 8611] AH00163: Apache/2.4.7 (Ubuntu) PHP/5.5.9-1ubuntu4.5 OpenSSL/1.0.1f configured -- resuming normal operations
[Sun Feb 08 21:06:03.204009 2015] [core:notice] [pid 8611] AH00094: Command line: '/usr/sbin/apache2'
[Sun Feb 08 21:06:07.213427 2015] [core:notice] [pid 8611] AH00051: child pid 8616 exit signal Segmentation fault (11), possible coredump in /etc/apache2
I believe this has to do with domain mapping as everything was working fine up until I activated it. Deactivating all plugins and reinstalling Wordpress does not fix the issue. I end up having to delete the database and do a full reinstall each time.
This issue affects the Network Settings page as well as the General Settings page for individual sites on the network.
Like I mentioned, this is the second site this is happening to and this time its on a fresh server. Any advice on how to fix this would be appreciated.