It occured now twice to me, that I could not sign in, because I got an infinite Castle-loading-Screen.
I get the passwordprompt where I fill in the password for the beta, but then I get stuck with the castle.
First time it occured to me with Firefox 50.0.2 on Win 10 Pro x64. I cleared the cache, cookies, tried to refreh several times. After some time it suddenly worked.
Second time, it was on a Ubuntu 16.10 with Firefox and Chromium. We changed to Windows 10 with Firefox and Edge. Didnt work. We deleted cookies, cleared cache and refreshed for like 10 minutes. Than it worked out of nowhere.
Note that both times, this happened on the campus of our university, beeing logged in in the local wifi. Maybe this caused the problems.
Sounds like an unstable connection with the metaserver. We're switching to https and secure websockets with the next release. If the problem persists after that, please report back.