@rose@pl.pinkro.se It's Mastodon doing its weird thing with relaying posts to other instances. The actor in the signature is mismatched with the activity, because Mastodon has to resign the request with a new key and fallback to verifying authenticity with LD Signatures which Pleroma doesn't support. You can ignore these warnings and blame Mastodon for not having an internal flag to stop spamming remote instances with requests that will always fail. What's more likely is that you had 410 HTTP response codes on this instance when it went dead, because Misskey craps out when that happens without doing that, and now Misskey treats this instance as dead.