I had a customer (18.104.22.168) who had some similar issues with users on a VPN. Screen sharing would not work properly from time to time, freezes, blank screens. I enabled network trace in the client and found that the client was disconnected from them server when on VPN.
I put this down as the cause as I doubted very much it would be the server.
The client can perform peer-to-peer application sharing which uses port 22222
. It could the case this port is not open via VPN or not reachable or natted etc.
In meeting policies you could disable the following policy to force all traffic through the server. This might just do it for assuming this is the problem.
Are your meeting servers accessible externally? What if the user drops the VPN and connects over the internet, can they successfully perform application sharing?