Ok I did try it on another testinstallation and the error is reproducable.
I will tell you what I did before:
After setting up CP 1.2 I went to this address https://github.com/ClassicPress-research/classic-elements for downloading the plugin.
There I said download as .zip and I got a file called ‘classic-elementor-master.zip’.
Than I go to the backend and install the plugin by uploading it.
Than I activate the plugin.
I have no errormessages or any hint that something went wront.
Than I open a page to edit with elementor but what I see are 2 big buttons:
Klicking one of these buttons will lead to a white page with a loading icon in the left upper corner:
Thats it. So I ask myself what did I do wrong?
I can send anyone who want to have look the login credentials.