Weird, same thing for me. My Bionic wouldn't connect, said "unable to connect to Verizon server." Even more annoying, we were on an 8 hour road trip, and I was planning on using that hotspot! But, now it seems to be working fine.
I actually suspect a real network problem, at least not something specific to Foxfi. I was unable to connect to Words With Friends a day or 2 ago. I was shown the "network required, retry" screen, even though I had 3 bars of 4G, and other internet things seemed to work.
Hopefully this was just a glitch, and not some nefarious attempt to find out who is using foxfi....