Hallo, Steffi...
Is it the case if Photoshop runs alone?
Is it the case if it is in Fullscreen?
I do not remember if rulers were showing, there's a preference about it to be checked.
Did you maybe check the Disable native canvas in preferences/Technology preview for another issue? It reverts Photoshop to an older code.
Please post the contents of Photoshop's Help>System Info... menu, so that we get to know your OS, its version, the exact version of Photoshop you are running, the date of the GPU driver, etc.