Hello People, I have a similar problem, change-cover-image does not work. This happens with every theme. I have two installations of wordpress, one works (installed locally) and a remote one in the web server which does not work.
Making test with buddypress style I installed plugins as follows:
BuddyPress Profile Tabs
BuddyPress – New UI
Both currently deleted. I cannot say change-cover-image was working before the plugins above.
What I can say is that some strange characteres are displayed before and after the select-file frame as follows:
Before:” /*Subir/*]]>*/
/*Hacer foto/*]]>*/
/* ”
After:” /*]]>*/ ”
I included the quotes. JIC my installation is in spanish.