550 Changes file must be signed with a valid GPG signature: Verification failed 3 times: ['General error', 'General error', 'General error'] : Permission denied.
Note: This error might indicate a problem with your passive_ftp setting.
Please consult dput.cf(5) for details on this configuration option.
...but it turns out that the upload is actually accepted and processed and built and published, etc. This is very confusing.
Interestingly, https://launchpad.net/bugs/757248 was marked "fixed in stable" exactly one month ago today. Perhaps it's the same problem that cropped up again.