Dan over at SoCoder suggested there's an "AToB" issue.

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Uncaught DOMException: Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.

Hmm..
This means that, somewhere along the way, one of the saved projects has become corrupt.
Whether it's the code itself or something else, I'm not sure, but it seems really odd that it hasn't saved properly.
I've rejigged a ton of code to account for glitchy AToB saves, so hopefully that fixes the issue, but I'm worried it might end up losing a project or two along the way.
Grrr..

To be fair, if the save is already corrupt, the project's already lost, and the new code won't have additionally lost anything new. It just now won't break the whole site if a project HAS become corrupt.

But what could corrupt a project?
I'm really not sure.
Perhaps it was mid-save when the browser closed, or maybe the site crashed during a save..
It's a very odd scenario.