Every few days I would be asked to validate my PRO License. If I tried while I was connected to my home wifi, it would fail. For a while, I would disconnect from my Wifi, validate the license, then reconnect. This got old pretty quick, so I decided to actually solve the problem. The problem ended up being that nzb360.com couldn't be connected to from my home network and that is the licensing server. To fix this, I just messed with the DNS settings of my router until I was able to connect to it. I'm not going to go into exactly what I did because my setup is fairly odd, just know that if you have the same issues as I did, ensure that you can connect to nzb360.com on that network. If you can't, fix that and try again.
I have been having this same issue, but mine is related to the VPN I use (Private Internet Access). It seems that nzb360 has blocked some if not all of the PIA servers. It's pretty frustrating considering the naure of this app.