This has come up for a few users, I believe. What I've found with one user so far is that they were using VPN software that was causing the lockout. I'm not entirely sure why, but the VPN IP Address being used didn't seem to work well with our firewall settings on the boards. It may be a dynamic IP issue or something else, but they were able to access the forums fine either with the VPN turned off, or by using the incognito mode of their browser.
If you're having problems, send me a PM and let me know if you're using VPN software to access the internet (the other poster was using Avast's VPN service).