おっけー！下記以降のバックトレースを消すことができました
2017-09-24T08:25:44.747905+00:00 app[web.1]: 45 TID-oi7w4 WARN: Mastodon::UnexpectedResponseError: Delivery failed for https://リモートインスタンス/inbox: https://リモートインスタンス/inbox returned code 500
2017-09-24T08:25:44.747961+00:00 app[web.1]: 45 TID-oi7w4 WARN: /app/app/workers/activitypub/delivery_worker.rb:17:in `perform'
https://github.com/zunda/mastodon/commit/0e93595ec1faca967fc2bbc056e0d12ce9a0bce3
github.comSuppress backtrace when failed to communicate with a remote instance · zunda/mastodon@0e93595mastodon - A GNU Social-compatible microblogging server
Pull requestしました
https://github.com/tootsuite/mastodon/pull/5076
github.comSuppress backtrace when failed to communicate with a remote instance by zunda · Pull Request #5076 · tootsuite/mastodonWhen we're sure that the trouble is on the remote instance, backtrace like below are a little bit too much. This change removes the backtrace except for the first two lines.
45 TID-ovbdu282o WARN: ...