I think I've just run into the same problem, though with Apache. If you view source on the page do you see the bulk of the html still but empty frames?
If so then the problem is most likely because of the following line in your nginx setup:
add_header X-Frame-Options DENY;
Setting this to SAMEORIGIN brought phpmyadmin back for me, this directive is preventing the page from being displayed in a frame in any context.
I couldn't find any errors in any of the logs tried.