loschle-gmx-de The specific component being rendered in the main-body area has font-awesome font-family not configured properly due to which the other font-family as seen for similar css attribute is also conflicting henceforth icon isnt appearing
Try add this to css file or in css field area of JA builder page settings:
.icon-step-forward::before{content: "\f0da";}
.icon-step-backward::before{content: "\f0d9";}
Also, The span classes like <span class="icon-step-backward icon-previous"></span>
do not have any icon to be shown which should be inherited through css attributes
You may also wish to contact the 3rd party extension team for the specific issue as the issue alongwith other issue as shown in your last screenshot is also apparently seen when we switch the template to default
Therefore, The icons to be apparently seen should come from font-awesome - the iconic font and CSS framework for which external css file for font-awesome is being called however, the icon properties havent been defined in specific css attributes