Does anything appear in your logs about the error? If so can you post it here?
socket.io 400 error polling
I've tried apache as well as nginx on a digital ocean server running Ubuntu 16.10. Using the default configuration for SSL listed on the nodebb site, the forums load properly but the sockets are never able to connect the user.
GET https://forum.serversite.com/socket.io/?EIO=3&transport=polling&t=Lfzci_0 400 ()
@Async0x42 I see the same issue with HTTP as well on a subdomain400 (Bad Request). Will report here if i find a fix