On Windows 11, after updating from 5.1.60, all of the Dorico buttons/icons were devoid of graphical content — including those in the panels, the top toolbar, the status bar at the bottom, in all the modes (ie Setup, Write), and those in the Steinberg Hub. In their place were empty squares. This was after I had rebooted the computer following the update procedure. I rebooted a second time with no change.
However, using the Repair option in Control Panel->Programs and Features has for now fixed the problem and the buttons are restored. Posting this in case anyone finds it helpful.
It sounds like someone else encountered something like this, but in the context of all program files first having been deleted during the update. In that case, the icons missing after reinstallation were restored by a reboot. See this post.