The most likely reason for a timeout is this: *) SSL_MODE_AUTO_RETRY is enabled by default. Applications that use blocking I/O in combination with something like select() or poll() will hang. This can be turned off again using SSL_CTX_clear_mode(). Many applications do not properly handle non-application data records, and TLS 1.3 sends more of such records. Setting SSL_MODE_AUTO_RETRY works around the problems in those applications, but can also break some. It's recommended to read the manpages about SSL_read(), SSL_write(), SSL_get_error(), SSL_shutdown(), SSL_CTX_set_mode() and SSL_CTX_set_read_ahead() again.
Kurt