Sentry stopped reporting errors

In /var/log/supervisor/sentry-worker.log, I see lots of these messages:

20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:19760ec9d8c94bf1819deb62e24014db:16')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:ccd81833afb54d19816b4b9187c51afb:14')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:b144338739fc498a872d7d6fc345f660:14')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:6d375e44b7c84af9a689aeda7efaf530:14')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:94fef35a29d04f62b60e0863a5798758:22')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:5b62390f78654241b5a9be7ee0d49dab:2')
20:31:36 [ERROR] sentry.errors.events: preprocess.failed.empty (cache_key='e:9092cfb5c6304b128b66cf6437e34182:16')

Did someone restart Sentry recently?

# sudo supervisorctl status
sentry-cron                      RUNNING    pid 79430, uptime 6:41:12
sentry-web                       RUNNING    pid 79426, uptime 6:41:12
sentry-worker                    RUNNING    pid 79431, uptime 6:41:12
root@sentry:/var/log/supervisor#