I was able to retrieve an older version of the MC29 folder that lives inside my user>Library>Application>J River folder from a backup. I replaced the existing folder with the backup version. When I launched MC29, I saw the same dialog window I attached to the initial post. This time, when I selected the default option of 'Reopen', MC29 launched and appeared normal.
I have nothing to base this on, but I wonder if the problem could be related to a 'Recently Imported' playlist. I have seen that playlist appear and disappear over recent months for reasons I do not understand. I tend to have it open in a tab all the time. Who knows if that means anything, but it might be something to look into.
I would like to know that this situation is understood at some point. It seems like there must be some issue that could be solved to prevent it from reoccurring.