I have actually discovered an additional related problem with just the Alt key alone, (which of course causes the problem with Ctrl + Alt). I understand that in Edit mode if you click between vertexes with Alt held down, an entire ring is selected. This does not happen for me. The Alt button does work for sure under other circumstances though, e.g. Alt + H to reveal previously hidden objects. No idea why it fails in edit mode like this. Any help appreciated.
