Hello,
I have seen this caused by a few different things, but usually it is caused by either Name Resolution.
Do you have a proxy? If so turn it off
ArcGIS for Server Relies heavily on the "LocalHost" variable. Ensure that localhost resolves to 127.0.0.1. Also ensure that your server name resolves correctly on the network.
Clear out your etc/host file from all entries (just comment them out) except for local host and try it.
What operating system are you using?
Thank you very much