liquify filter does not let me perform any action, why?

[miriamb88886161]

Hello everybody, I'm trying to use the liquify filter, but every time I open it and try to do an action it won't let me and a little window pops up saying "The backdrop settings are preventing the preview from showing images changes". I changed the backdrop setting, but it still does not work. how can I fix this?

[c.pfaffenbichler]

Please post meaningful screenshots.