That warning 'PHP Warning: Cannot modify header information' is causing white page. Good news is that your forms are posted and changes saved, but later html drawing fails for some reason. You need to at least identify why / what's causing it. Disable all plugins and try again. Then, gradually, one-by-by enable them back until it happens. Maybe you did some changes on your own? What have you changed/installed recently? Some incompatibility between code may exist.