Beginner having problems with Admin

1. Question: First it worked very well but then I saved a page and now I always get those Errors.
   I have the following problem when opening Grav Admin Plugin after Login:
   E_COMPILE_ERROR - Cannot declare class Grav\Common\Page\Medium\GlobalMedia, because the name is already in use
2. Question: I am using the antimatter theme. How can I import and use a banner like in the demo?

Sorry for my bad english and thanks for your reply,
Philipp

It is very unlikely that this error is caused by saving a page. Has anything else changed? Did you install some plugins or have any other weird behaviour in Grav?