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[raina]

...because Preview only works in Quick Reply and not in the full post editor. The XHR response is an HTML snippet with an error about session timeout, and logging out and back in doesn't fix it.

[Saga Musix]

Yeah, something's really quirky about the session timeouts (you will also often get them when staying on a site too long before posting), but I have no idea yet why it happens (and it doesn't help that I'm not familiar with nginx).