Actually a Rioter from Support is the one that told me that told me to try a VPN among other things when I encountered this problem:
Ichiban Ramen here with Riot Tech support and today I'll be assisting you. I'm sorry to hear that you're having issues with proceeding with access to the PBE site. There appears to be an issue with the PBE link on the site so we'll go about doing this with a workaround. Use the following steps and see if it helps:
- Head over to the main League of Legends website: http://na.leagueoflegends.com.
- Log in by clicking on the login button on the top right.
- Copy and paste this url into the address bar of your browser: https://pbesignup.na.leagueoflegends.com/en/pbe
- Continue further with your PBE access.
If that does not work for you, test your connection out using a VPN. Here's a list of some VPNs that are available on the Internet:
CyberGhost VPN
Hotspot Shield
UltraSurf
WTFast (This isn't a free VPN service, but it does have a trial period that's a bit more comprehensive than the rest. Recommend trying this first.)
Let me know if you still have issues and I'll be happy to help further.
So using a VPN is perfectly fine.