Chrome version (54.0.2840.59 m (64-bit) causes weird behaviour when posting replies. It trims space as you type while in WYSIWYG editor mode, so it appears that your space bar is not working.
Current workaround is to switch from WYSIWYG editor mode to Source mode. You can do this with the upper left button in the reply area:
If you're not sure what mode you're in, hover over this button to see the tooltip popup. If you're in Source you'll see "Switch Editor to WYSIWYG Mode" and if you're in WYSIWYG mode you'll see "Switch Editor to Source Mode." It's easy to toggle between the two.