Server Offline in Live Discover

In the mcsclient.log on the computers in the failure state, do you see any errors around this stage:

2021-03-15T17:37:05.871Z [ 4624: 7972] [v4.13.154.0] INFO Establishing push connection
2021-03-15T17:37:05.875Z [ 4624: 7972] [v4.13.154.0] INFO [push]: [connect] trying server mcs-push-server-eu-west-1.prod.hydra.sophos.com/ps
2021-03-15T17:37:05.875Z [ 4624: 7972] [v4.13.154.0] INFO [push]: [connect] trying direct connection without a proxy
2021-03-15T17:37:05.875Z [ 4624: 7972] [v4.13.154.0] INFO GET mcs-push-server-eu-west-1.prod.hydra.sophos.com:443/ps
2021-03-15T17:37:06.096Z [ 4624: 7972] [v4.13.154.0] INFO 200 : sent=0 rcvd=0 elapsed=220ms
2021-03-15T17:37:06.096Z [ 4624: 7972] [v4.13.154.0] INFO [push]: [connect] using server mcs-push-server-eu-west-1.prod.hydra.sophos.com/ps without a proxy (peer address x.x.x.x)
2021-03-15T17:37:06.102Z [ 4624: 7972] [v4.13.154.0] INFO (async) GET mcs-push-server-eu-west-1.prod.hydra.sophos.com:443/.../0089d6b7-83c0-149f-2a0e-188e5237df41
2021-03-15T17:37:06.612Z [ 4624: 1968] [v4.13.154.0] INFO (async) 200 : connection established
2021-03-15T17:37:06.612Z [ 4624: 1968] [v4.13.154.0] INFO (async) 200 : chunk=1 rcvd=6 conntime=510ms

Maybe compare them to the working ones.  Any different?

I double checked the sophos list with the websites in my FW, all are there, only hydra.sophos.com stays unresolved. I have a rule that allows all Internal Sources HTTPS traffic to Sophos recommended websites. This does not work, there is a timeout when it tries to push.

If I allow all traffic to all sites, then the push is succesful.

I double checked the sophos list with the websites in my FW, all are there, only hydra.sophos.com stays unresolved. I have a rule that allows all Internal Sources HTTPS traffic to Sophos recommended websites. This does not work, there is a timeout when it tries to push.

If I allow all traffic to all sites, then the push is succesful.

Is it an XG firewall?  Any clue as to why it's being dropped?  What rule is it hitting?

It is still an UTM. I fixed it temporarely by adding extra IP adresses. I noticed those adresses were being blocked at the time I was installing InterceptX. By adding more and more (now 9 in total) of these adresses Live Discover started to see my Devices. Next on my list is to determine which of these 9 adresses are actually needed.

Thank you for your confirmation, Please keep us posted and let us know if you have more queries about this. I have marked your response as the answer of this query.