How can I find conflicting keyboard shortcuts?
I actually found the culprit...

In order to find the error I tried several other numbers with ⌘ + Shift and found ⌘ + Shift + 6 triggering a screen recording interface I hadn't seen before.

Opening this interfaces options I found it originated from CloudApp and there in CloudApps "Keyboard Shortcuts" I found it (see image).



CloudApp must have integrated the shortcut with it's recent update.

I removed the shortcut and now Safari is working again as it should :)