if anyone's curious about the unspecific complaints here, it transpires that the actual complaint is synapse had a bug where it some endpoints were called with

if anyone's curious about the unspecific complaints here, it transpires that the actual complaint is synapse had a bug where it some endpoints were called with a trailing slash when the spec said it shouldn't have one... it got fixed in https://github.com/matrix-org/synapse/pull/4840 https://github.com/matrix-org/synapse/pull/4840 and https://github.com/matrix-org/synapse/pull/4935 https://github.com/matrix-org/synapse/pull/4935.