"Internal Server Error" when trying to add a phone number

### Steps to reproduce

1. Open the Android Element app (found at F-Droid)
2. Go to Settings (Einstellungen) > General (Allgemein) > Mail & Phone (E-Mail und Telefon) > Add Phone Number (Telefonnummer hinzufügen).
3. Enter a valid mobile phone number in the format +49016205768453 and click proceed (Fortsetzen)
4. See an error message "Internal Server Error" popping up.

Identity-Server vector.im

### Outcome

#### What did you expect?
I expected the app to accept my valid phone number entered in the suggested format.

#### What happened instead?
Internal Server Error message, phone number was not added.

### Your phone model

Moto X4

### Operating system version

LOS20

### Application version and app store

F-Droid, Element 1.5.30

### Homeserver

matrix.org

### Will you send logs?

Yes

### Are you willing to provide a PR?

Yes