We apologize for the instability of composr.app and appreciate your patience. We are working on the statistics addon and trying to find an optimal way to store and render data. Unfortunately, we have yet to find a solution that can handle the traffic (and therefore, tens of millions of statistical records) of composr.app. We're working hard on one.
#1399 - Black Screen while trying to view embeded video
However, your first issue is likely user error and would need more info. Please feel free to take it over to the forum. It is likely you don't have the right codec in the mp4 file (it needs to be h.264). The mp4 file extension doesn't imply any particular file format (codec), this is a common point of confusion. Flash/HTML5 needs h.264.
I have fixed your second issue in #1400.
However, your first issue is likely user error and would need more info. Please feel free to take it over to the forum. It is likely you don't have the right codec in the mp4 file (it needs to be h.264). The mp4 file extension doesn't imply any particular file format (codec), this is a common point of confusion. Flash/HTML5 needs h.264.