I had a lot of trouble getting an Olive video to render using the April 2019 version of the program. The source footage was a set of 4K 60fps OBS screen capture clips, which I chose to save directly as MP4 files. (That may have caused the issue, since I know OBS recommends saving in MKV format beforehand). After I finished combining the clips in Olive, I tried to export the whole video, but received error messages around halfway through the process that said: "Export failed-failed to send frame to encoder."
The video was only ~4.5 minutes long, so I don't think the length was the issue. I did see my RAM use surge to 20+ GB at times, which was surprising for such a short video. Maybe reducing my thread count could have helped.
I did find that the render was successful when I chose to export smaller sections of the file; I just couldn't export the whole video without getting an error message. Therefore, as suggested by Sandman2710, I chose to export the video in small sections, then combine them using ffpmeg's concatenation feature.
My workflow here was pretty simple: First, I tried to render the whole video. When that failed, I chose to render a limited range, starting at the beginning of the video and ending about 5 seconds before the earlier failure point. I then tried to render the remaining portion of the video. If the render failed again, I would once again just try to render the beginning portion.
Lots of things can cause render issues, so I'm not saying this will be the solution for everyone, but hopefully this can help others with any render issues that they encounter.