- Re-order stages in registration flows such that msisdn and email verification are done last. ([\#5174](https://github.com/matrix-org/synapse/issues/5174))
- Fix a bug where the register endpoint would fail with M_THREEPID_IN_USE instead of returning an account previously registered in the same session. ([\#5187](https://github.com/matrix-org/synapse/issues/5187))
- Prevent registration for user ids that are to long to fit into a state key. Contributed by Reid Anderson. ([\#5198](https://github.com/matrix-org/synapse/issues/5198))
- Prevent registration for user ids that are too long to fit into a state key. Contributed by Reid Anderson. ([\#5198](https://github.com/matrix-org/synapse/issues/5198))
- Fix incompatibility between ACME support and Python 3.5.2. ([\#5218](https://github.com/matrix-org/synapse/issues/5218))
- Fix error handling for rooms whose versions are unknown. ([\#5219](https://github.com/matrix-org/synapse/issues/5219))