Issue resolved! My server shard was on a blocklist with my ISP, totally unrelated to anything Bluesky was doing.
Today I became unable to log into my original bluesky account - I can password-reset, but then I get this opaque error. Weird because I can set up a _new_ account on bsky.social but for some reason I can't log into my old one. Open to suggestions about how to recover from this.