Hey Torsten,
Well, try that. Go to components/com_hotspots/models/forms/marker.xml and find this code:
<field name="hotspotText" type="editor" class="inputbox required"
label="COM_HOTSPOTS_FIELD_DESCRIPTION" description="COM_HOTSPOTS_FIELD_DESCRIPTION_DESC"
filter="JComponentHelper::filterText" buttons="article,image,pagebreak" required="true"/>
Try to remove the required="true". Let me know if this works then. I think it should, but I haven't tested it.
cheers,
Daniel
P.S. This is for the frontend only - in the backend you have another .xml form to modify.