xmpp-js unable to join a MUC from matrix

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I am trying to join an XMPP MUC:
room name: test
MUC domain: con.xmpp.example.com
Matrix domain: matrix.example.com

So the room name should be:
#_xmpp_con.xmpp.example.com_test:matrix.example.com

if I understand the wiki correctly.

If I try that I only get an error in riot that it is not a valid room name.
Continuing anyway gives an error in the synapse logs (epexted "!" prefix)

The relevant config file part (registration.yaml) is:
```
namespaces:
users:
- exclusive: true
regex: '@_xmpp_.*'
aliases:
- exclusive: true
regex: '#_xmpp_.*'
rooms: []
```

Any hints would be appreciated.

Start with the registration.yaml namespaces shown in the issue, then reproduce the alias in Riot and inspect the corresponding Synapse log error. Compare the documented XMPP MUC room naming with the configured alias pattern; done means the room alias is accepted and joining the MUC succeeds.

Written by the indexing model from the issue text.