Users with accounts on matrix.org can communicate among themselves but cannot decrypt messages from users with accounts on any other servers. The error 'unable to decrypt message' appears in Element, and 'message could not be decrypted due to missing key' in Nheko.
Meanwhile, there are no key exchange problems between users on any other servers.
In other words, matrix.org has isolated itself, forcing everyone else (those who set up their own servers or registered on small company servers, for example) to create accounts on matrix.org to avoid losing contact with the majority of people on the network.
The developers are silent as spies: https://github.com/matrix-org/matrix.org/issues/2483
Although the problem has been observed at least since the end of July.
The problem was tested on different clients (both on PC and Android), in various operating systems, with different servers, under VPN and without, with different residentional providers. The /discardsession command was tried — it's definitely not something that broke on my end personally.
I believe that amid the fight against end-to-end encryption in England, Germany, and France, authorities have simply approached them and asked to create a walled garden to prevent people from spreading across uncontrolled servers.