tinc - Mar 2019 - Sometimes can't establish tinc connection due to authentication timeout?

Mar 11 11:24:52 box1 tincd: Trying to connect to svr1 (a.a.a.a port 443)
Mar 11 11:24:52 box1 tincd: Connected to svr1 (a.a.a.a port 443)
Mar 11 11:25:02 box1 tincd: Timeout from svr1 (a.a.a.a port 443) during
authentication
Mar 11 11:25:02 box1 tincd: Closing connection with svr1 (a.a.a.a port 443)
Mar 11 11:25:02 box1 tincd: Could not set up a meta connection to svr1

the log as above, I did a tcpdump of that, and the TCP handshake went well, but
it appears during the key exchange, both side can’t receive the packet(contain
the key) from each other...