I had also the same error:

The server network address "TCP://<server>:5022" can not be reached or does not exist. Check the network address name and that the ports for the local and remote endpoints are operational.

While building about 20 mirrors. And the strange was that I was able to build the first 10 mirrors, then this error appeared. But if I tried again then I was success. Than fail again and success again, or not.

First I suspect for the network....

Then I realized the following:

On the mirror instance I had two SQL instances running. One of them ate 20Gigs of memory, the other one left only 600MB...

What do you think which one was the one I tried to set up for mirroring?

Once it got some memory (2-3 gigs) it was easily build the mirrors, and the error was gone...

Andor