This module is a workaround for servers that try to do s2s
authentication with certificates and SASL EXTERNAL, but do not send
correct stream headers. Notably Openfire versions since 3.7 or 3.8.
With the plugin
installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_s2s_auth_compat
For earlier versions see the documentation for installing 3rd party modules