Whenever I select Appearance --> Customize and the custom sidebar opens I get the log-in pop-up "Session expired. Please log in again. You will not move away from this page." I can still use the custom bar and publish the changes, so I'm not actually logged out, just can't see the preview in the custom section. Trying to re-login doesn't work, the log-in window keeps popping up. This first happened when I changed my blog's theme from Twenty Eleven to Twenty Fourteen. I had never seen this before. Anyone knows how to fix this?