Thanks, @spikeyxxx! The error message disappears but reappears as soon as I click on “Update Denpendencies”. Furthermore, the red line under “pose.bones[“CTRL_armSpline.001”].constraints[“Copy Rotation.001″].influence” in the “Drivers List” on the left side of the image above stays. It refers to a bone “CTRL_armSpline.001” which no longer exists (as far as I can see) because it’s one of the bones which are either on the left or the right side of the rig and therefore have been renamed with a suffix “.L” or “.R” using Blender’s renaming functions.
