The error message is:
This page contains content or formatting that is not valid. You can find more information in the affected sections
If you put control with associated validation controls in the master page - asp.net Login control for example, the page validation couldn't process the page correctly and page couldn't be save when it is in edit mode.
put your asp:Login tag in EditModePanel and set PageDisplayMode="Display"
This code will render the Login control only in display mode, not in edit.