POST https://matrix-client.matrix.org/_matrix/client/r0/join/%23libera%3Alibera.chat errors with 502

This issue has been migrated from [#14596](https://github.com/matrix-org/synapse/issues/14596).

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### Description

It is impossible to join `#libera:libera.chat` from matrix.org

`POST` requests to `https://matrix-client.matrix.org/_matrix/client/r0/join/%23libera%3Alibera.chat` error with Cloudflare's standard 502

I don't know how long this has been going on, but people have been complaining about it in `#irc:matrix.org` and `#libera-matrix:libera.chat` for about a month.

### Steps to reproduce

- Try to join `#libera:libera.chat` from matrix.org

### Homeserver

matrix.org

### Synapse Version

1.73.0rc2 (b=matrix-org-hotfixes,34fa1276a4)

### Installation Method

I don't know

### Database

postgresql

### Workers

I don't know

### Platform

n/a

### Configuration

_No response_

### Relevant log output

```shell
n/a
```

### Anything else that would be useful to know?

_No response_