Welcome to the forum.
This is a known problem, which Google has failed to document despite us developers having notified them in Video content supplied *inline* causes server fault since May 16th.
The video content can not be posted as inlineData, which is what the request you show is doing. That is guaranteed to generate a 500 error return code. To use video content, you have to use the File API. There is a technical reason, the File API performs preprocessing on the video, sampling it at fixed time intervals and saving the sampled images in a set. That preprocessing is missing when the video is transmitted inline.
Hope that helps!