- Fix long-standing bug where sending email notifications would fail for rooms that the server had since left. ([\#9257](https://github.com/matrix-org/synapse/issues/9257))
- Fix bug in Synapse 1.27.0rc1 which meant the "session expired" error page during SSO registration was badly formatted. ([\#9296](https://github.com/matrix-org/synapse/issues/9296))
- Fix bug introduced in Synapse 1.27.0rc1 which meant the "session expired" error page during SSO registration was badly formatted. ([\#9296](https://github.com/matrix-org/synapse/issues/9296))
- Assert a maximum length for some parameters for spec compliance. ([\#9321](https://github.com/matrix-org/synapse/issues/9321), [\#9393](https://github.com/matrix-org/synapse/issues/9393))
- Fix additional errors when previewing URLs: "AttributeError 'NoneType' object has no attribute 'xpath'" and "ValueError: Unicode strings with encoding declaration are not supported. Please use bytes input or XML fragments without declaration.". ([\#9333](https://github.com/matrix-org/synapse/issues/9333))
- Fix a bug causing Synapse to impose the wrong type constraints on fields when processing responses from appservices to `/_matrix/app/v1/thirdparty/user/{protocol}`. ([\#9361](https://github.com/matrix-org/synapse/issues/9361))
