IMPORTANT: It turned out that this question couldn't really be answered based on the info I had provided, and it wasn't possible for me to provide enough info to make it answerable. Tom provided the best steps for debugging this type of issue in a normal situation so I'm giving it to him, but in my case the issue being that some changes to core that are not released yet, were causing problems in the way that Secure form processing worked. Turning of secure form processing until this issue is remedied was the solution that worked for me.
I'm having an issue in my ee2 install in which most often when I edit or create an entry in the control panel and click submit, the control panel homepage is loaded and the content is not saved. If I hit back in my browser and resubmit, the change is saved on the second try.
Has anybody else seen this, and does anyone know what the fix is likely to be?