Dreamhost moved the entirety of the site data to a "DreamObject" backup storage and hence it's no longer online.
Their reasoning for doing that is that Tiddlyspot site consists of "personal backups" and hence violates their terms of use. I presume this is because the "backups" folder stores previous versions of your TiddlyWiki.
(There was also a bug where the number of site backups would sometimes grow unbounded rather than being pruned down to a fixed number of revisions. So there is a reasonable case for Dreamhost to be annoyed by that.)
Bad News
The future of Tiddlyspot is unclear. I'll need to negotiate with Dreamhost and I'm not sure how communicative they're going to be. Maybe if we agree to remove all the site backups Dreamhost would put it back how it was, but I don't expect it to be done quickly.
Good News
Your site data is presumably safe and sound in a DreamObject, so if for some reason the Tiddlyspot version was your only copy there's a good chance it can be recovered.
Question?
Tiddlyspot was pretty cool back when it was new, but it has aged badly and (I'll confess) been neglected by its owners for many years. For this reason, the idea of gently retiring Tiddlyspot has been considered. Maybe we'll hang on to the domain name for when/if Daniel & I come up with a new, modern, secure version of Tiddlyspot. That said, I don't have a good sense of how many people are using it actively now and how many would miss it if it were gone. So, if you are seriously impacted by Tiddlyspot being unavailable, please let me know how and why.
Update, Oct 25: I have a support ticket open at Dreamhost to request access to the data. The ticket number is #9255564.