something is indeed wrong. it could be that the process is too intensive considering you are rotobrushing a compressed footage for a very very long duration. try this: transcode your original mp4 footage to a lossless format like Quicktime Animation, then reimport, then replace the footage in the timeline, then try to freeze.
still one minute is very very long. are you sure you need the whole minute rotobrushed? you might want to check the help files to see you are doing it right: After Effects Roto Brush, Refine Edge, and Refine Matte