As i said i can use the UI / Inventory functions normal (right click to see information about an item etc)
but just to be sure i unplugged the mouse, and even without the mouse he keeps swinging the hammer.
Also if it would be a mouse issue it would affect other games or applications aswell (what is not the case).
If i go into the menu and assign both mouse buttons to the left hand, than i can use the left hand normal with both mouse buttons while the right hand keeps swinging.
(but there is no mouse button or keyboard key assigned to it)
This all feels so buggy and unlogic that i think i gonna wait for another game update for now
Edit: i uninstalled the game via steam, deleted the whole folder of the game to get rid of any rest config or temp files.
Installed new, started the game and: problem still there.