In fiddling around with this, the problem seemed to go away, and then came back. I was about to conclude it must be an intermittent thing (having something to do with the website). But it bothered me that a clean install seemed to fix it. So I experimented with
imdbconf.cfg—disabling it, restarting the program, re-enabling, etc. I soon found I was able to cause the problem to return. I haven't been able to pin down exactly what's going on, but it's consistent enough to conclude the problem has something to do with how the configuration settings are saved and recalled. I put it that way because I haven't been able to cause or fix the problem by changing the configuration within the program. The change only occurs after the program is restarted. I don't know if the specific setting changed makes any difference, but the one I've had the most "luck" with (i.e., causing something to happen) is
Download tags from the first page only. Even then, it's not consistent, and I have yet to see any pattern in whether it's set to on or off. It's making a change, restarting the program, and running the plugin—twice—that seems to make the difference. Sometimes.
After getting this far, I decided to see if I could get it in a stable state by
not changing the configuration. No such luck. Regardless of which way that setting was set, and even if it was last changed to fix the problem, restarting the program causes the problem to return. It makes me wonder if this is happening to everyone, and is just to subtle to notice. Fortunately, it happened to in the "working" state when I ran the plugin overnight to update my entire database for the new votes and top 250 data.