Paul Bijnens wrote:

It is not an ack, but the client receives the "noop" packet again.
This means that the server never got an "ack" from its first request.

Aha.

Is there a firewall in between?

Nope.

Do you have multiple interfaces on that computer

Yes! Both of them have other interfaces on private IP address ranges.

(may the reply got sent using the other interface,
and not recognized by the server). etc. etc.

I'll go away and investigate - thanks!

--
Illtud Daniel                                 illtud.daniel AT llgc.org DOT uk
Uwch Ddadansoddwr Systemau                       Senior Systems Analyst
Llyfrgell Genedlaethol Cymru                  National Library of Wales
Yn siarad drosof fy hun, nid LlGC   -  Speaking personally, not for NLW