We have seen odd frame rates of source files cause this error message after production. When you produce the MP4 let's use the following. File > Produce and Share > Custom Production Settings > MP4 > Video settings tab. On that tab change the frame rate from automatic to 30 and see how that MP4 turns out.
We have also seen producing to a network drive or folder that syncs with cloud storage cause this behavior. We recommend all source files and the production folder be on the local hard drive.