CalDAV not syncing like before

For some reason the only way that I can get my mailbox calendars to sync with my Android phone is by using DAVx5.

For many, many months I was able to use the built-in CalDAV sync tool with the calendar app Calengoo. The last several weeks I was having issues getting it to sync in a timely manner and now after deleting the sync account and creating a new one, I'm not able to even connect at all.

I first thought it was an issue with Calengoo, so I tried installing Business Calendar 2 and configured its CalDAV interface. It also will not work correctly.

With Calengoo, I get the message "Could not load your calendar/task list". I'm able to get a little farther with Business Calendar 2. It will at least find my calendars and present them in a list, but then that's it. None of my calendar events will be displayed in the calendar.

This is very frustrating. I'm assuming that it has to be something that changed on mailbox's end, as things were working correctly for the longest time and no update was done to my calendar apps.

Has mailbox changed something on their end, so that it now only works with DAVx5?