Resolved. For roughly ten minutes, from about 22:15 to 22:25 UTC, the website and the API returned errors.
Requests during that window failed outright. They did not return wrong answers, so there is nothing to re-check: anything that got a response before or after the window was correct.
No data was lost. Submissions are accepted and processed normally again, and nothing in the database was affected.
The cause was a failure on one of the machines running the service. The database moved to another machine automatically, which restored service.
Public data file downloads at data.acoustid.org were unaffected throughout.