I get a QuickTime Error -50 sometimes when trying to render.
What I've done to fix that has worked several times but I'd like to confirm it with others.
When I get that error I switch to exporting a Master. When the error happens again, this time it gives me a frame number.
I switch the Preferences setting for the timecode window in the middle bar to display in frames and I got to that frame number.
From there I will do one of a couple of things. My theory is that the render for the clip there is corrupt and you need to force a new render for that clip.
I will either select the clip > Shift F to identify it in the Event Browser > Replace the clip on the timeline. That didn't work yesterday but blading the clip did. Possibly if you select the culprit clip and fiddle with Color you might force a new render for the clip.
Yesterday I had three clips at the frame number of the error and by disabling (V) each one in turn I identified the culprit clip (by doing a short range render with an IN and OUT around the suspect frame).
I can confirm that this export error is often related to corrupt or wrongly connected render files. Your workflow seems logical as it creates new render files for the problematic ones. Many times I delete all Project and Event render files before sharing a project. This does not affect export times.