The old Sonos Bridge is the likely culprit here, as it is widely known that it’s now old power supply voltage can fluctuate and cause connection issues .. Maybe try setting it aside and just leave the one ‘other’ Sonos device wired, I’m assuming that’s a standalone speaker and not a sub or HT surround?

The other possibility here is duplicate IP addresses, which could be tested by unplugging all Sonos devices from power, then rebooting the router. Once the router comes back up, plug in the BRIDGE first, then the other Sonos devices. 

Duplicate IP addresses can occur with either ‘wired’ or WiFi connected systems, since the error is in the router. 

Thank you for the help.

I did try to cycle the power of everything. Didn’t help. 
Removing the bridge helped once to have the beam and Sonos One back “online”.

weird thing was that after while having all units back online (including the bridge), the 2 units and the bridge disappeared again.

I did try the same procedure, but didn’t work this time. After while, out of nothing, all units came online again.

What is a huge coincidence (or not), is that I have been running the same setup for about an year without any issues. All pretty solid here. As I turned on one unit that required update, all units were also updated to the latest version and just after finishing the update, the issue started.

but, as said, it resolved by itself??!

Thank you so much for helping me.

I would just leave the Bridge disconnected and powered off and leave a standalone speaker wired direct to the router instead, or even try running the devices on your routers WiFi signal.

See these two links:  

https://support.sonos.com/s/article/3235

https://support.sonos.com/s/article/3209