Regarding Watchout 5.5.2 Production on Windows 10, you can't start a Display computer's (Win 7) timeline using the WATCHOUT Production application on Windows 10.
If you press spacebar/play on WATCHOUT Production on Windows 7, the timeline on the WATCHOUT Display (Win 7) starts.
If you press spacebar/play on WATCHOUT Production on Windows 10, the timeline on the WATCHOUT Display (Win 7) does not start. The preview on the WATCHOUT Production does start though.
Using telnet on Windows 10 to control WATCHOUT Display (Win 7) works properly.
What you describe is a network problem, not a Windows problem per se.
You Win 10 system is not sending its UDP broadcast packets
(and the TCP/IP is getting through ok as expected).
Check for multiple NICs in the Win 10 system, if there are,
disable all but the WATCHOUT NIC and see of the problem disappears.
If that does not fix it, take a second look at firewalls settings, network switch DOS protections, etc.
Its all a bit academic, as Win 10 is not really compatible with WATCHOUT 5 anyway,
but as you discovered, it will mostly work with a bit of care on your part.