When we try to upload a video file directly to a Canvas content page and add captions to it, the window disappears. We've tried from a few different computers (PC, Mac) and browsers. The problem relates to .srt files and .vtt files. Here's an example:

https://youtu.be/scDU7DwTH2E

For the time being, we are uploading videos to YouTube, applying captions, and them embedding those videos within Canvas pages as our workaround.