The question in this post is related to White screen of death on post save only, but it is a different question. Actually it's two questions on the same issue.
Background: Three of us were working on an emergency site rebuild/switch to WordPress over the weekend. Two people were on content, I was on setup. I started a child theme, based on Twenty Sixteen. When I was deleting the unneeded themes that installed by default, I accidentally deleted Twenty Sixteen (the parent theme) instead of Twenty Fifteen. I immediately re-installed Twenty Sixteen but it seems that subsequent problems may be been related to this error on my part.
The Symptoms: We saw a gradual degradation of the dashboard. First we started getting the white screen of death on adding or updating a post. The posts were being added and updated, but we had to go back to the list to see it. This degraded to other functions until finally, two of use could not log in.
The Fix: the problem was solved when both Twenty Sixteen and the child theme were replaced with Twenty Seventeen. Then Twenty Sixteen and the child theme were added back in and Twenty Seventeen and the child were deleted.
The Question(s): where in the code or database did the error occur? Why would a theme error cause a problem with the dashboard?