That error happened to me too, and I created a new account, and saved the recovery key to a password manager.

Matrix frequently asks for the recovery key (too often, if you ask me, but it's for 'security' I guess). But since I have it saved in the password manager, I never have the unable to decrypt problem any more.

I've now had at least two matrix accounts for about a year, which haven't had that problem.

I'm not sure if this will solve the issues you're having, but I thought it might be helpful to somebody to post this anecdote. If you were having trouble with matrix in the past, it might be time to try again.