What’s probably happening is a DM is attempting to tag me which sends me a message but because messages are E2EE I don’t have the key to decrypt the message that tagged me between two other people. At the very least that seems to be one of the ways this error message is prompted. There may be more scenarios where your key is expected to decrypt something but can’t.

This is my best guess, someone more knowledgeable than me should step in 🫡