Unable to Sync Calendar to External CalDAV Clients

I've been having an issue going on a week or so now where, no matter the OS or app used, I've stopped receiving calendar updates and can no longer sync with my mailbox.org calendar.

Here are my setups:

- Win10 (latest with extended security updates) and Mozilla Thunderbird

- Android (latest GrapheneOS on Pixel 6) and OneCalendar

- iOS (on iPad) with native Apple Calendar App

- MacOS (Tahoe) with Mozilla Thunderbird

Things I've tried:

- All were previously properly configured with bespoke 2FA app passwords, but I disabled 2FA on my mailbox.org account to test each platform and app using my main password - no dice.

- I've tried uninstalling and reinstalling OneCalendar on Android. No change.

- I've logged out of and back into both Thunderbird clients on MacOS and Win10. No change.

- I submitted a ticket with the mailbox.org team, but haven't had a response as of yet - they let me know response times are extended due to an influx of users. Understandable, but very inconvenient.

Mailbox.org calendar still works on the mailbox.org site, but there's basically no way for me to check my calendar without logging in on a web browser - this defeats the purpose of using the built-in calendar.

Is anyone else having this issue? Has anyone else figured out how to make it work? I don't see any bespoke settings on mailbox.org's portal that would have changed to prevent access from external CalDAV apps.