I had the same issue: Series 3 with LTE, iPhone on 11.3, Watch on 4.2.3 and I could not get
Messenger to load. I too found the same website above (and others) and I followed their advice: I
uninstalled/re-installed the app on iPhone and Apple Watch 3-4 times, powered off and force
restarted numerous times, I even updated the Default Replies in the iPhone Messenger Profile
settings, without any initial success.
However - I eventually got Messenger to work, but only for one day. It had to do with the Default
Replies. Not sure why. All I did was modify EVERY Default Reply, exit out of Messenger on the
iPhone, then went back into Messenger, then watch finally connected to Messenger and began to
load. I then reset all the Default Replies back to stock.
This was yesterday, 2/26/18. Now today, 2/27, I just tried to open Messenger on watch and it is
back to "spinning" mode as I am typing this.
I just now modified all the Default Replies...did the same things as above and now Messenger is back
to working on the Apple Watch. Seems very sketchy. I'm going to keep the Default Replies custom
and see if this keeps it working beyond one day.