Main UI values are not displayed

I had more or less the same problem.
SSE connection is blocked in some way, by firewall or part of the virus scanner. for example, it worked well on my android devices but not my windows 10 desktop.

https://community.openhab.org/t/oh3-value-of-items-not-visible-main-ui-windows-10-but-android-is-ok/113376/5

Good luck figuring out which piece of software on your client device is responsible for blocking SSE traffic / connection