This trouble come from conflict with the x-frame options of your server. Look at whether the plugin succeeded in writing this following lines in the .htaccess of your wordpress appli if you can:
# BEGIN Lf-hiker plugin <IfModule mod_headers.c> Header set X-Frame-Options SAMEORIGIN </IfModule> # END Lf-hiker plugin
It resolve the trouble for me.
You can find more information about this trouble in wordpress support Multiple ‘X-Frame-Options’ headers with conflicting values