Confluence Proxy Error Reason : Error reading from remote server

Since this morning I have trouble accessing my confluence. It's on an Ubuntu 12.04 server.

The error itself is this

Proxy Error

The proxy server received an invalid response from an upstream server.
The proxy server could not handle the request GET /xxx/xx/xxxxx.

Reason: Error reading from remote server

I use an apache with reverse proxy

my apache2 errorlog shows this :

(70007)The timeout specified has expired: proxy: error reading status line from remote server

My apache config is this :

<VirtualHost *:80>
    ServerName www.confluence.xxxxx.xxx
    ServerAlias confluence.xxxxx.xxx

    ProxyRequests Off
    <Proxy *>
        Order deny,allow
        Allow from all
    </Proxy>

    ErrorLog /var/log/apache2/error.log

    ProxyPass / http://xxx.xxx.xxx.xx:xxxx/ Keepalive=On
    ProxyPassReverse / http://xxx.xxx.xxx.xx:xxxx/
    <Location />
        Order allow,deny
        Allow from all
    </Location>
</VirtualHost>

I saw some people fixing this by adding the Keepalive=On but for me it didn't help. I restarted apache2 of course.. no success.

Any Ideas? Let me know if you need more information, I can give all you need.

EDIT :

I have to add that the official confluence page is offline or has troubles right now and when the problem started. Can that be related? I mean it's somehow connected, since you can install the updates and addons through interface.

EDIT 2 :

one of our users says that it crashed after he imported and xml document using the menue..