Billing Control Panel hangs on order placement, which leads to Service Unavailable error after Apache timeout (30 seconds by default).
/usr/local/bm/log/generic_worker.log contains the following errors repetatively:
Operations Automation is not available
OA 8440 port is available from BM, and it is possible to execute API request from Billing Application Node to OA Management Node over HTTP:
# curl -firstname.lastname@example.org http://192.0.2.2:8440/RPC2 ...output is received...
API request over HTTPS hangs:
# curl -email@example.com https://192.0.2.2:8440/RPC2 ...hangs here...
SSL is enabled on BA side:
select SSL from "PEMOptions"; SSL | 1
and disabled on OA side in in System > Settings > Public API.
Enable SSL for Public API to allow secure communication between Operations and Billing.