As of deep last light (cest), the error was identified and tchncs started to inform other servers about the events they were missing out on. It wasn't processing one of the outgoing queues correctly due to a well hidden typo in a configfile – causing one of the federation senders not identifying itself correctly.

You may run into a few recent messages that can't be decrypted, but otherwise everything federation related should be back to normal.

Sorry about this mess and thanks to everybody who has helped in identifying the issue.