That message means your computer/router/network is blocking UDP protocol or losing it. The server has no control over this.
More than likely it has to do with the port range the Mission arena is using. It could be related to packet loss from another program intercepting the data, such as an anti-xxx program.
Those are about the only circumstances where that message would occur.
We have been waiting to hear from anyone else if this is happening for them and have not heard anything yet. I tried it from home and it works fine from there as well as from the office (to be expected).
If it was server related, it would be happening with great consistency for many, many people, if not all of them.
I do not know what type of Internet connection you have. There are, potentially easy things to try depending on the Internet connection type and the hardware involved. For instance, by-passing the router is easy to do for some connections and might point out something.