Cannot change themes of newly created sites "Access denied"

When creating a new site i cannot change the default theme (grayed out) and a message appears something like "access denied". What's wrong? The first 6 sites where created succesfully and have a modified (OOB) site theme applied, all sites I create from now on cannot have the default theme modified. I checked any check-out settings but couldnt find anything obvious. What's wrong? I didn't use any custom code or such, just clicked around like an Admin does ;)  I thought about some check-in/out issue, but couldnt see anything. Someone HELP! please!

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I am a SharePoint Infrastructure Engineer, focussed on Administration and Installation of SharePoint Server environments.