fresh installation, no upload possible

I installed the plugin but cant upload nothing. After saving the document in backend, the website tries to open a ‘random’ frontend page with additional /&mdocs-cat=mdocuments and produces error message as that page doesn’t exist. When returning to the backend, there is no saved mepmphis document. I did’nt change anything in configuration, just installed the plugin and tried to upload a file…

*latest wordpress version*