Graphical output in notebook cells disappear [solved]

I was working with a notebook that was successfully outputting plots. I literally stepped out for lunch and when I came back all graphics cells were empty. When I reran the cells there was no longer an graphical output. When I looked at the server logs, it looks like something happened after an autosave. The error message is below. I’m just looking for any guidance as to where to start looking for the problem because error messages do not mean anything to me.

The culprit here ends up being caused by notebook extensions. A link and discussion is here.

So far my experience is that while some of the extensions are really handy, they frequently cause problems. Perhaps not worth the instability and definitely not worth it if you are using notebooks as a de facto IDE.