Ever since the forums went down and were restored, my account will randomly log itself out. Say, I log in to Computer A. I'm still signed on from yesterday. I then use Computer B, where I signed in yesterday as well, but it's logged me off for some reason. I know for a fact I logged on on the computers that I am no longer logged in on.
This can occur when there are many cookies (little files in a computer that store personal data) holding the same log-in session across multiple devices. It does this on my phone for BZPower and many other sites. There isn't a way to fix this to my knowledge. It's never caused me much trouble.
The same thing has been happening to me since the restoration: if I stay on one page for too long (though "too long" is pretty subjective) then when I change pages I'll find that I've been randomly logged out.
Not sure if this is the same issue or not, but there aren't any other topics for this problem.
You should probably clear your BZPower-related cookies from all the devices you're trying to log in to and give it another go. I have no problems being logged in on four or so devices, logging off and some and still being logged in on others, etc.