The title says it all: I opened an Office365 site on Sharepoint Designer 2013 and then tried to edit some .aspx pages. When trying to save, I get "Access Denied" error.

If I try to edit and save an .html page, I get no errors. If I try and rename the .html page to .aspx I get acces denied. So it seemds like editing and saving aspx pages is disabled somewhere.

I have made sure that: I am a site administrator and Run Custom Web Scripts was enabled last week from the Office 365 admin site