fatal error: cannot use object of type ...

I installed Drupal 8 Beta 3 on my Hoster and after activating Gratis 2.0 (gratis-8.x-2.x-dev.tar.gz) I get the following error:

Fatal error: Cannot use object of type Drupal\Core\Page\HtmlPage as array in /drupal/themes/gratis/gratis.theme on line 18

It is basically a vanilla install except 2 things which have to be changed, otherwise Drupal 8 is not working on the hoster:
/root/.htaccess

# Don't show directory listings for URLs which map to a directory.
#Options -Indexes

# Follow symbolic links in this directory.
#Options +FollowSymLinks

/root/sites/default/files

# Turn off all options we don't need.
#Options None
#Options +FollowSymLinks