Session expired, please login again
I also had this problem, plus when I did login and tried to change anything it threw an Invalid CSRF token error. Got the solution from this thread;
https://sourceforge.net/p/adminer/discussion/960418/thread/17b3972a/
I edited adminer and explicitly added in the session save path for the server;
ini_set('session.save_path', '/tmp/');
Old question, but answering for others with same problem who end up here.
Make sure nginx has write permissions to the php session storage directory. Just had to do this on my CentOS box because a php upgrade set the session storage root:apache and changing it to root:nginx did the trick.