Mazi wrote: 1. Did you update your Limesurvey version recently?

No, I didn't.

Mazi wrote: 2. Please upload a link to an activated sample survey for testing.

You can see it right on the welcome screen. --> arbeitszufriedenheit.net/limesurvey/index.php/215916/lang-de

Try adding this to your template.css file:
.ui-button-icon-secondary
{
display: none;
}

It works, but only for the "next" button. The "< previous" button is still double spaced.

Well, then I need another link which also shows the "previous" button

oh, right, there it is

arbeitszufriedenheit.net/limesurvey/index.php/215916/lang-de

Nope, that's not the correct link, the survey ID is missing so all I get is a 404 error.

you're quicker than my edit

arbeitszufriedenheit.net/limesurvey/index.php/215916/lang-de

This is the extended CSS code including both:
.ui-button-icon-primary, .ui-button-icon-secondary
{
display: none;
}

Works perfectly, thank you very much!

flink wrote: Works perfectly, thank you very much!

You're welcome. You should think about just getting a nice template in general. Check www.limesurvey-templates.com/index.php :-)