Unable to reply to private message

Each time I try to reply to a private message I get :

Forbidden

You don't have permission to access /private.php on this server.
Apache/2.4.7 (Ubuntu) Server at ubuntuforums.org Port 443

I haven't changed any permissions, yet I was able to reply earlier in the same private message thread.
Meanwhile, I have been able to post on the forum.

While trying again to send a reply to the private message, the following was displayed on the screen :

Apache/2.4.7 (Ubuntu) Server at ubuntuforums.org Port 443

then :

Forbidden

You don't have permission to access /private.php on this server.
Apache/2.4.7 (Ubuntu) Server at ubuntuforums.org Port 443