I get similar results on elph130. The propcase is 170 instead of 171 for this camera (propset 6, verified by checking prop value after changing in menu). Romlog attached. I haven't looked at what this assert actually means, but I expect that changing movie res requires more than just setting the propcase. I don't think this is a bug in CHDK code, it's just how the canon firmware responds to having that value changed.
Recording and stopping video with click'video" over PTP works fine if I don't change the propcase.
The only alternatives I can think of are to set this using key presses through the Canon menus, or perhaps a UI property
http://chdk.setepontos.com/index.php?topic=10624.0