Tiles rendering problem

Like it was said in one of my question on stackexchange, I'm trying to configure my own tile server in CentOS 6 following this tutorial.

I had some issues that have been solved, everything is working perfectly in my previous configured VM : I can render my tiles in a distant computer in QGIS, it's very fast and no bugged.

I was thinking that it was going to be the same during the configuration on a "real" server, but I have some new problems...

In fact, http://myserveradress/osm_tiles/0/0/0.png is working. I can also get an access to my server from QGIS using QuickMapService like in my VM :

Coasts lines are present before a zoom

BUT beyond a certain zoom level in a country (when it's supposed to render some streets/places/etc) I get this :

And the screen is white...

Also, a strange fact from France rendering (country that I had tried to get into my database to finally stop the process) :

Africa (that is uploaded in my DB) is supposed to be rendered...not France

I was wondering if it was coming from the server performances so I used the command

render_list --all --socket=/var/run/renderd/renderd.sock -z 0 -Z 10

in order to pre-render some tiles and decrease the rendering time but I also get an error like :

I searched on the net in order to find some clues but most people do not find answers to this problem.

I uploaded all Africa continent during the last night and I swear I reused the same process as in my VM configuration...

Does someone have an idea ?