Hi again Zulf,
This might not be due to the plugin directly, but after testing the new theme and this plugin on a new site, I decided to revamp one of my older sites with Twenty Fourteen and related plugins too. However, while doing this apparently something went wrong.
Now, whenever I select Appearance --> Customize and the custom sidebar opens I get the log-in pop-up "Session expired. Please log in again."
I can still use the custom bar and publish the changes, so I'm not actually completely logged out, just can't see the preview in the custom section. Just trying to re-login doesn't work, the log-in window keeps popping up.
Any idea how to fix this?