i had the same problem today and i figured out the it was created by the tag <WebPartPages:SPWebPartManager runat="server"/>
wich was out of the form tag!!!
Some of the default masterpages of sharepoint have this control outside the form tag (example:BlueBand.master, BlueTabs.master) wich results in the problem you have. I think only the default.master is correct.
So if you have this problem verify if in your masterpages you have the control <WebPartPages:SPWebPartManager runat="server"/> inside your form tag.
Hope to help you.