Mysteriously the problem went away !
There was a non standard dynamic port rule for tcp
netsh int ipv4 show dynamicport tcp
showed port range to be
Protocol tcp Dynamic Port Range
Start Port : 1024
Number of Ports : 64511
The default for windows is:
Protocol tcp Dynamic Port Range
Start Port : 49152
Number of Ports : 16384
This was corrected and the default range set for ipv6 as well and also for udp - but at t the same time, also applied an optional windows update
This brought windows 10 Pro version 1909 from OS build level 18363.752 to 18363.753
After making those 2 changes and a reboot the problem went away
What is baffling is that uninstalling that update and also re-instating the non standard port range made no difference - the tcp port exhaustion was no longer happening
Microsoft did have a bug in this area that was resolved
Addresses an issue that causes all Transmission Control Protocol (TCP) dynamic ports to be consumed. As a result, network communications will fail for any protocol or operation using dynamic port
But this was already applied before and was subsequently superseded when the windows 10 version went up from 1903 to 1909
My only guess at this point is that the windows update from OS build 18363.752 to 18363.753 corrected something that the uninstall of the update left in place
