Plone 5.2 Theme upload

Can somebody please help me with Plone 5.2. I tested it on a Ubuntu based system and in a docker container but if i would upload a Diazo-Them as zip file and click "Upload" i'll be forwarded to and i get displayed this message: Excuse me, but the website you tried to reach is not available here. Please use the listed links to find what you are looking for. And the site do not have any styles without the plone-toolbar-container. And if i reload the page without sending the form-data the theme isn't uploaded and all is normal.

PS: I don't know anything about using docker and Plone, but since nobody else has answered:

Are you sure it is a Plone problem? I have had the same with docker containers and Wordpress due to rights/upload size (etc).