QR login fails with "Invalid rendezvous URI" when MAS is enabled

[CyberWarr]

Steps to reproduce

1. Configure Synapse with OAuth delegation to MAS (msc3861_enabled: true)
2. Configure MAS v1.14.0 as the OIDC provider
3. Open Element Web and navigate to login screen
4. Click "Sign in with QR code"
5. Observe error message

Outcome

What did you expect?

QR code should be generated successfully. After scanning with Element X, the device linking process should complete.

What happened instead?

Error message appears: "QR-код не поддерживается. Поставщик учетной записи не поддерживает вход на новое устройство с помощью QR-кода."

Browser console shows:
Error whilst generating QR code Error: Invalid rendezvous URI
at async Y.generateAndShowCode (LoginWithQR.tsx:112:36)
at async Y.updateMode (LoginWithQR.tsx:88:27)

Operating system

Server: Ubuntu 22.04 Client: Windows 11

Browser information

Chrome 146.0.0.0 Edge 146.0.0.0

URL for webapp

Self-hosted. Element Web version: 1.12.13

Application version

Element Web: 1.12.13 Olm: latest

Homeserver

Synapse 1.151.0rc1 with MAS v1.14.0 (OAuth delegation enabled via msc3861)

Will you send logs?

Yes