I had also the same error:
[color=#FF0000]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.[/color]
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...