Access to tile server forbiden

I am currently building a web application and use osm maps.
I need to extract some specific area sometimes and i use a static map php script hosted on my system.

It was working for some times and suddently it fails with error : failed to open stream: HTTP request failed! HTTP/1.1 403 Forbidden

Can someone point me out where is miss the thing ?

I think i respect osm policies (referer, user-agent, etc.).
I can give more detail if needed (ip, domain…)

thanks for your help.