unable to add email address to my account

*This issue was originally created by [**@p4tpr0**](https://github.com/p4tpr0) at .*

### Background information

- **Dendrite version or git SHA**: dendrite-0.13.4
- **SQLite3 or Postgres?**: postgresql13-server-13.12
- **Running in Docker?**: no
- **`go version`**: n/a
- **Client used (if applicable)**: element-web-1.11.47 & Element Version 1.11.47 (1.11.47) on macOS

### Description

- **What** is the problem: unable to add email address to my account (in parameters)
- **Who** is affected: me as a client of this instance (currently the only user of this instance)
- **How** is this bug manifesting: message in Element GUI. Error reads "MatrixError: [400] Untrusted server '' (https://myhost/_matrix/client/v3/account/3pid/email/requestToken)"
- **When** did this first appear: at my first try to add my email address to my account parameters

### Steps to reproduce

- connect to the server either with desktop or web Element
- browse to account parameters, scroll to email address
- fill in an email address and submit ("Add")

a popup appears that reads:
```MatrixError: [400] Untrusted server '' (https://myhost/_matrix/client/v3/account/3pid/email/requestToken)```

In the log file:
```time="2023-11-07T16:50:50.963513546Z" level=error msg="threepid.CreateSession failed" func=github.com/matrix-org/dendrite/clientapi/routing.RequestEmailToken file="github.com/matrix-org/dendrite/clientapi/routing/threepid.go:83" error="untrusted server" req.id=zuvRVVioUaos req.method=POST req.path=/_matrix/client/v3/account/3pid/email/requestToken```