While I was signing up for PM, I had to attempt at least 5 times for both myself and my wife's account before I succeeded (I did them on separate days to make sure one of us always had a working phone, so I couldn't remember the magic combination of things I had to do to make it work the first time). But in the end, though, I realized the reason for the failure is because I use 1Password for my passwords. That always caused the form to claim that the password didn't match the password requirements. (For those that are programmers, 1Password sets a 'data' attribute on the password field, and somewhere in the JavaScript that checks the password requirements, it flips out if any 'data' attribute is set no matter what the suffix is.)
I also got complaints from Chrome that there was JavaScript being served insecurely, and it turns out the pages are loading JavaScript from HTTP instead of HTTPS URLs. I had to manually tell Chrome to load those scripts and ignore this issue.
