It's the template/theme you are using, just like with your other problem. It is not compatible with the Limesurvey version you are using.

Switch to one of the templates/themes that come with your version of Limesurvey and this problem (and the other one) should go away.

Thank you a lot for your answer.

How can I switch the template/theme?

Usually in the setting for the survey you can change the template. In LS 3.x (should be more or less the same for other version as well) you can find it in "Settings" -> "General survey settings" -> "Template". www.limesurvey.org/manual/General_settings

To help you further, just like mentioned in your other post, we need some basic information:

What LS version are you using?
What template are you using?