Hi Guys,
After reading the suggestions and tried different ways I still cannot figure out how to solve this thedious problem.
Here you can find the "Ruby console" output.

Closing UI
Starting Export
Error: #<NameError: uninitialized constant LumenRT2015::VueExportGUI::Main>
e:/program files/e-on software/lumenrt 2015/lumenrt/export/skp/ruby/lumenrt_ui.rbs:477:in showUI' e:/program files/e-on software/lumenrt 2015/lumenrt/export/skp/ruby/lumenrt_menu.rbs:139:in lumenRT_exportModel'
e:/program files/e-on software/lumenrt 2015/lumenrt/export/skp/ruby/lumenrt_menu.rbs:535:in block in <module:LumenRT2015>' SketchUp:1:in call'

All the best.
Patrick