Synapse 1.15.1 is a bug fix release to fix two bugs introduced in 1.15.0.
The first prevented users of certain clients (including Riot) to add or remove third party identifiers such as email addresses.
The second caused Synapse not to start when configured with certain password auth providers.
Sorry if you have been bitten!
Get 1.15.1 from github or any of the sources mentioned at https://github.com/matrix-org/synapse/blob/master/INSTALL.md.
Changelog since v1.15.0