HI!
Seems to be, http_async client is not compatible with http2 servers.
I'm having like
kamailio_1 | 15(20) INFO: http_async_client [http_multi.c:238]: debug_cb(): [cURL] Connection #0 to host
relay-api.pbxservice.com left intact
kamailio_1 | 15(20) ERROR: <core> [core/parser/parse_fline.c:264]: parse_first_line(): parse_first_line: bad message (offset: 11)
kamailio_1 | 15(20) ERROR: <core> [core/parser/msg_parser.c:681]: parse_msg(): ERROR: parse_msg: message=<HTTP/2 404
kamailio_1 | date: Thu, 14 Nov 2019 18:31:34 GMT
....
kamailio_1 | 15(20) ERROR: <core> [core/parser/msg_parser.c:337]: parse_headers(): bad header field [(null)]
kamailio_1 | 15(20) ERROR: <core> [core/parser/msg_parser.c:1274]: get_body(): failed to parse to end of headers
But plain http_client worked good (not async for sure)