EasyBlog Composer datetimepicker not working with template Simpli

Dear Stackideas,

I switched from "beez3" template to "Joomlart Simpli".

Now, the datetimepicker widget in EasyBlog composer does not work anymore. The button is enabled and reacts, the opened widget is empty however.

See screenshot: the datetimepicker for creation date is only an empty "bar".

Which functionallity is needed for this?
Simpli is a CSS template only, do I need another script?

How can I debug this? There is no error message ein the console and no Joomla Debug message.

Kind regards
Ralf

Notes:
- switching back to beeze: datetimepicker works again
- editing in backend (default template): datetimepicker works