I'm sorry but I have no input to give you on what would be causing this, but I want you to know that you're not alone. I too experience this when I try to connect to Ministry's servers. It seems I'm the only one that can't get in when we have a team build going on.
When I host a dedicated, it shows the ping as --- for me and yet people can still connect. I used to be able to host without that happening.
My only thought is that maybe you have port 28000 forwarded and you have a dynamic IP address, so sometimes the port is forwarded to you and sometimes it isn't. Try assigning yourself a static IP address and forwarding port 28000 to it.
I've had this problem with people unable to join my server.I restarted my computer and it worked.
tthe worst part is that once and a while they can get on.