It's definitely Alt-click on the (old, Gnome 2) Ubuntu box I've got here.
Alt-drag has been used for as many years as I can remember in the Linux world for dragging windows, which can cause it to clash with Inkscape's use of Alt. My usual advice is to disable it in your window manager's keybindings (which will generally disable it for all applications), or hold down the Super (Windows) key first, then hold Alt, then click or drag (so it becomes Super-Alt-click). On most window managers the latter approach is sufficient for it to not interpret it as an Alt-click for moving the window.
FWIW, the Alt-drag feature of Linux can be a godsend when you're forced into a low-resolution screen for some reason and have to deal with dialogues that are too big to fit.