Q: Messages Server Clients Unable to Connect

I have an unsolved problem with iChat/Messages, which (while otherwise working OK over Bonjour) suddenly lost the ability to send files over our local network, multiple versions of the OS 10.11, 10.10 and some 10.6. We recently purchased a new Mac Mini and have set it up as a file server. I was hoping that by using the Messages Server in OS X Server App (5.1.5) that we might regain this functionality. I have set up the Messages feature on the server according to the tutorial, and the clients too, however, the clients are unable to connect via Jabber, with the following alert available from the miniature lightning icon in the buddies list:

"Messages lost the connection to the Jabber account “myname.local”. A network error occurred."

My host name is server.myname.private

My computer name is server.myname

Messages status is: available on your local network at server- myname.local

Permissions are: all users, all networks

Archiving is on

Server federation, well I have tried every combination while troubleshooting

Standard certificate set for Messages in certificates

Clients Messages applications account is set to Jabber (enable this account is ticked)

Clients account names are username@myname.local

Clients passwords are set to their user passwords

Clients account server settings are default settings (server is myname.local, port 5223, use SSL ticked)

Automatically find server and port, warn before sending password insecurely and warn before sending files insecurely are all ticked, use Kerberos v5 for authentication is un-ticked as per default.

I have tried every combination of account names: username@server.myname.private, username@server.myname.local, username@server.myname, as well as the correct (I think) username@myname.local, none of them work.

I have trashed and rebuilt client preferences: com.apple.iChat.AIM.plist, com.apple.iChat.Jabber.plist, as per some googled suggestions.

Can anybody help? This is driving me around the bend.

Oh, while I am at it, does anybody know why if you have SMB and AFP enabled on the file serving side of things you can connect to the server using the Go>Connect to Server menu item (or Command+K), but not through the AFP share in the Finder Sidebar Shared list, all you get is a "connection failed" message. The SMB share will connect fine through the Sidebar when it bothers to appear, which is not always. Turn off SMB and the Finder Side Bar method works fine for AFP shares again.