is there a problem with the LibreSSL build ?

reqconn: Could not send GET request. (Function not implemented [38])

Yes, looks like some SSL functions are not implemented for libressl and retest fails with ENOSYS.

I think ssl_err=2 means SSL_ERR_WANT_READ

https://github.com/openssl/openssl/blob/master/include/openssl/ssl.h.in#L1206

This means that the operation has to be repeated ...
similar logic is done in tls_tcp.c

tls: SSL_verify_client_post_handshake error: Bad address [14], ssl_err=2

LibreSSL only stubs the SSL_verify_client_post_handshake API and returns always a error if used.

TLS 1.3 post handshakes are an extension, and HTTP/2, for example, explicitly forbids them (because of the multiplexing). So it's a very rarely used feature. We should add a feature/flag detection for this, I think.