Unable to log into non-matrix.org account; "well-known not available".

[AidanofVT]

Steps to reproduce

1. Install Element on Android; open the app.
2. Tap "SIGN IN"
3. Tap "EDIT" next to the server field, which shows "matrix.org" by default.
4. Delete "matrix.org" from the editable field on the "Select your server" page which is now shown; the "NEXT" button becomes greyed-out.
5. Enter the name of my private homeserver; "NEXT" button becomes available again
6. Tap "NEXT"; a warning appears: "well-known not available".

Outcome

Expected: proceed to a log-in page which will allow me to log into my privately-hosted account.

Actual result: unable to proceed.

This happens in all downstream apps which I have tried, although its presentation is sometimes a little different: they don't all show the error. It is not present in any non-Android Matrix clients, as far as I can tell.

Your phone model

Galaxy s10e

Operating system version

Android 11

Application version and app store

Element 1.6.44 from F-Droid; various other stores and downstream apps.

Homeserver

Private matrix_synapse_py3, version 1.124.0

Will you send logs?

Yes

Are you willing to provide a PR?

No

[bro2020]

I confirm.
In Synapse version 1.138.0, the /_matrix/client/r0/login endpoint returns 404 if MAS is enabled in homeserver.conf.
This endpoint must be handled by the MAS server.
But it returns json:

{"errcode":"M_UNRECOGNIZED","error":"Unrecognized request"}

[bro2020]

The problem was in the incorrectly configured Compatibility layer:
https://element-hq.github.io/matrix-authentication-service/setup/reverse-proxy.html
(If this is not set at all, Element Classic has no chance to authenticate.)

My problem was that I had to adapt the example from the documentation to HAProxy.
I made a mistake in the acl:

acl old_auth_paths path_reg -i ^/_matrix/client/.*/(login|logout|refresh)$

But right:

acl old_auth_path path_reg -i ^/_matrix/client/.*/(login|logout|refresh)

Because there are requests like /_matrix/client/r0/login/sso/... and others.

[AidanofVT]

I am using nginx. I'm an amateur, so I'm not really able to follow what you're saying, but, with your insight as a starting point, eventually the AIs I use were able to identify the problem.

My understanding is that Matrix Android, and its derivatives, are more current in their expectation of a ".well-known" response from the server. Other clients have fallbacks if they don't get an appropriate response, or maybe they just use other methods from the get-go. So, the solution for me was to put some listeners in the nginx configuration which intercept the ".well-known" queries and reply with confirmations about how to access the server.

If this sounds reasonable, let's go ahead and close this issue, but if I'm being mislead then please turn me in the right direction.

[bro2020]

@AidanofVT
Of course, if you use the software as described in the documentation and follow it, everything will work for you.
Of course, the /.well-known/matrix/client and /.well-known/matrix/server endpoints must be present.
It seems you can also use DNS TXT records, but i'm not sure that most clients and server implementations will support this well.