Internal Server Error 500 upon activation of a WordPress plugin

My WP installation generates a Internal Error 500 every time I activate the BuddyPress plugin.

This is the actual error:

[Sat Mar 28 19:12:28.102868 2015] [fastcgi:error] [pid 10447] (104)Connection reset by peer: [client 192.168.1.1:55431] FastCGI: comm with server "/var/www/clients/client1/web4/cgi-bin/php5-fcgi-*-80-site.eu" aborted: read failed, referer: http://ift.tt/1CwmoFs

[Sat Mar 28 19:12:28.103055 2015] [fastcgi:error] [pid 10447] [client 192.168.1.1:55431] FastCGI: incomplete headers (0 bytes) received from server "/var/www/clients/client1/web4/cgi-bin/php5-fcgi-*-80-site.eu", referer: http://ift.tt/1CwmoFs`

How should I solve that?

P.S. The strange thing is that before, when the server was running Ubuntu 12, I had no troubles activating and using BuddyPress. Then something possessed me and I decided to upgrade the OS to 14.04 and as a result the site became inaccessible. Finally, I figured that it was BuddyPress that was causing the issue, removed it from the plugins folder and the site went back to working. Now every time I try to re-add BP, I get the error above.