Temp Fix: I got my Error msg problem to go away! Basically, I simply changed Import Limit for the calendar with the error to "Past 2 Years", and now no error. I changed setting back to its original "Everything" and the error returns. Actually, I found that any choice, other than Everything, results in no error.
Programmer: I think there is a problem with the dropdown tool for the Import Limit. While I was troubleshooting, I got the dropdown to display the list twice (see attached image). When I picked from the top/first list, I even got the field display to go blank. I'm guessing that the date range being queried from Google is blank or corrupted - thus the error message. I sure hope my temp fix helps others in the meantime. I love this app, but the missing calendar entries were killing me.