It turned out that it's not working only in Chrome browser. I
was able to sign up from Firefox. Also sign in and activation
doesn't work in Chrome. It looks like Chrome absolutely is not
supported on rubygems.org :)
I haven't heard any more issues like this lately. If you're
still seeing signup bugs, PLEASE PLEASE let us know! I'm not 100%
sure this issue is dead but we haven't heard any reports, and after
having several people make fake accounts, no errors happened.