Responsive Composer Sidebar empty

Hallo,

I wanted to create a newsletter layout with the responsive drag & drop composer but when I click on the composer, I only see an empty sidebar.
So I can not see any tools there.

I’m using the Themify “Ultra” theme with the Themify drag & drop builder on a woocommerce shop and wanted to integrate a newsletter with this awesome plugin.
Before the problem appears I tried it out on a testserver and everything was working fine. So I deactivated all wordpress plugins I installed for the shop and I also deactivated the my child theme but the problem is still there.

Could someone please help me and take a look over this bug?
Thank you!

Eric