daVinci Resolve: Split-clip shortcut on German keboard
It seems like this key simply doesn't work on German keyboards. 

It's probably because you have to press Alt-Gr to get the "\" sign on a German keyboard, and the software doesn't accept that additional key.

You can, however, redefine this function to another key. In contrast to some documentation, this cannot be found in the project settings, but directly in the first main menu entry "DaVinci Resolve|Keyboard Customization".