So, I guess I can marked this one as solved. I found a way around the issue, and I’ll detail below.
I was using public calendars with their link provided in the OP. Last night, I had some insomnia, so I decided to play around with the issue. I deleted the instances, deleted the plug-in, and reinstalled. I recreated the instances, and unfortunately, I still got the same HTTP 400 error.
Next, I tried going down the road of creating credentials. This should not be needed for public calendars, but I figured it might be worth a shot. With my credentials, still no luck. I then imported those calendars into my account. I then went into the settings for each calendar. I took the “Calendar ID” under Integrate Calendar, and put it into the “Calendar ID” on the instance. Progress, sort of. Now I’m getting a HTTP 403 error. It would retry several times, before finally telling me no events for the day. Even after it finally settles down, there is an event for the day, so it doesn’t appear reliable. I also went into the variables, and the error is in there an no events for the day. I looked up the 403 error, and it’s specific to Google Calendar where the Calendar is trying to be accessed too many times.
So, started over again. I deleted the instances, because I couldn’t remove the keywords under variables. It told me that the keyword was mandatory, even though the documentation says it could be blank . I couldn’t make it blank, so starting over was the best course of action.
My end result that is working flawlessly now is going back to the Integrate calendar setting under my Google Calendar for the ones I imported, and the last entry says “Public address in iCal format” with a https://calendar.google.com/calendar/ical/xyz12345 type address. It says “Use this address to access this calendar from other applications.” This works without any issues.
I know Stuart has moved on, so I felt documenting this issue for others may save some headaches.