API answer return 503 Service Unavailable

Hello,

I have a CWP - WiseCP API connection that was working fine until recently. Whenever a client make a new order from WiseCP, the account is being successfully created on CWP but then the API answer "503 Service Unavailable". Going back to client area in WiseCP i see the order is still in the cart, the amount is paid, and the service is activated.

My webserver is  NGINX-Varnish-Apache.

Any help would be much appreciated. thanks