Unfortunately the Dialog for the Section Planes doesn't react to Windows/Preferences/Workspace Reset Workspace. But you can try it.

As for the alt spacebar thing you need to be very specific.

Create a section plane then hold down Left Alt, not Alt Gr if you have one, and tap Spacebar once

Then tap the default letter for move in your language if it isn't english. You can check this with any window active by doing the Alt Spacebar thing and seeing what shows in the little dialog that appears, the correct letter will be underlined.

Tapping m will activate an icon of four arrows

Tap any arrow key will attach the four arrow icon to the lost dialog

Finally allowing you to move the mouse without clicking until the dialog appears on screen, at which point you can click to drop it.