I think this related to the issue discussed in
this post, also it happens with the recent 4.5.5p4.
Unity probably changed a behavior of some sort which broke anchoring inside the editor, I tried investigating and made some dirty fixes, but its still not behaving normally as it should, it seems its caused by mCam in UIPanel being null when worldCorners geter is called but didn't have the time to dive deeper into the code hoping it'll be fixed by either Unity or Aren soon.
A quick workaround is to edit anything in the code to trigger a recompile which gets everything into position, but hitting the play button messes it again ...