I logged this issue with support and they sent me here:
2 different PCs, Windows 7 & 10. Same network. Has been working like a dream for months. Now as of about 9 days ago, we could still log on, see leader boards etc (all with names and active changes) but there were no riders.
Nothing on our network had changed between when we started using Zwift and a week last Saturday. As of Wednesday this week riders are back - at least on the PC.
When I try and login from a different network on my Mac (which which Ive also used several times in the past at multiple locations to run Zwift) Im now facing the exact same issue - no riders on course, but active leader boards.
Once we finish riding we can upload rides to Strava etc. so we clearly have a working network. Three machines, two different Zwift accounts, 2 different locations / networks. Same problem.
Is there anything we can provide to help diagnose this issue further?