It took some digging with the debugger, but with enough breakpoints I was able to track down the cause. Your calls are failing because the imgifinfo function fails (it is called by readgif>read_multiframe_gif which is called by imread>call_format_specific_reader). It doesn't return an error, but it doesn't determine a backgroundcolor for the image.
The image has transparent pixels and no background. This is normally not an issue, but the disposalMethod is set to RestoreBG. This is the root cause of your issue. A less strict reader might inject a default background (MSpaint picks black), but the reading function in Matlab doesn't test for this case. I don't know enough of the format specification to know if Matlab is too strict, or that your gif file is simply incorrect.
current_frame(transparent_pixels) = background_color;