This incident has been resolved.

We observe that publishing video content to Instagram fails intermittently due to an issue at Instagram end.
We will partner with Facebook/Instagram team, and we will keep you posted when we hear back from Facebook contact or observe any updates through the public bug.

We are currently investigating intermittent errors with publishing video content to Instagram via our Direct Publishing feature.
As a workaround, please use Instagram Mobile Handoff for publishing video content. We will keep you posted when we have more information on this.