Verify Session with MAS not working

[xundeenergie]

Describe the bug
I set up MAS (matrix-authentication-service) with keycloak backend.
When i login with element X i can login.
Next step, verify Session:
I get the notification on another client (Element, Schildichat, both on Android and Linux desktop)
But element X stuck forever in "Waiting for another Device"

When i try to login with element oder Element-web, the verification-process works fluently with MAS and keycloak-backend also on the same device.

To Reproduce
Steps to reproduce the behavior:

1. Login with OIDC on Element X (MAS is configured for synapse)
2. Verify Device/Session
3. Accept verification-request on another device (Element/Schildichat...)
4. Wait forever on Element X to get the next screen.

Expected behavior
Verification on Element X working.

Screenshots

Desktop (please complete the following information):

Smartphone (please complete the following information):

• Device: Samsung A53
• OS: Android 14
• Browser: --
• Version: --

Additional context
Matrix-synapse 1.114.0 with integrated sliding-sync
matrix-authentication-service: v0.12.0
Identity-Provider: keycloak

When element x tries to start the verification-process for the new session i get this in homeserver.log

2024-09-17 09:48:00,744 - synapse.api.auth.msc3861_delegated - 338 - ERROR - POST-4041 - Failed to introspect token                                                                                                
Traceback (most recent call last):                                                                                                                                                                                 
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/http/client.py", line 425, in request                                                                                                       
    response = await make_deferred_yieldable(request_deferred)                                                                                                                                                     
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                     
twisted.internet.defer.CancelledError                                                                                                                                                                              
                                                                                                                                                                                                                   
During handling of the above exception, another exception occurred:                                                                                                                                                
                                                                                                                                                                                                                   
Traceback (most recent call last):                                                                                                                                                                                 
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/api/auth/msc3861_delegated.py", line 336, in get_user_by_access_token                                                                       
    introspection_result = await self._introspect_token(token)                                                                                                                                                     
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                     
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/api/auth/msc3861_delegated.py", line 227, in _introspect_token                                                                              
    response = await self._http_client.request(                                                                                                                                                                    
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^                                                                                                                                                                    
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/http/client.py", line 442, in request                                                                                                       
    e.args[0],                                                                                                                                                                                                     
    ~~~~~~^^^                                                                                                                                                                                                      
IndexError: tuple index out of range                                                                                                                                                                               
2024-09-17 09:48:00,747 - synapse.http.server - 782 - WARNING - POST-4041 - Not sending response to request <SynapseRequest at 0x7fc35013cdd0 method='POST' uri='/_matrix/client/v3/keys/query' clientproto='HTTP/1
.1' site='8008'>, already disconnected.                                                                                                                                                                            
2024-09-17 09:48:00,856 - synapse.http.server - 782 - WARNING - POST-4042 - Not sending response to request <SynapseRequest at 0x7fc306162d90 method='POST' uri='/_matrix/client/v3/keys/upload' clientproto='HTTP/
1.1' site='8008'>, already disconnected.                                                                                                                                                                           
2024-09-17 09:48:01,109 - synapse.api.auth.msc3861_delegated - 338 - ERROR - POST-4049 - Failed to introspect token
Traceback (most recent call last):
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/http/client.py", line 425, in request
    response = await make_deferred_yieldable(request_deferred)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
twisted.internet.defer.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/api/auth/msc3861_delegated.py", line 336, in get_user_by_access_token
    introspection_result = await self._introspect_token(token)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/api/auth/msc3861_delegated.py", line 227, in _introspect_token
    response = await self._http_client.request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.11/site-packages/synapse/http/client.py", line 442, in request
    e.args[0],
    ~~~~~~^^^
IndexError: tuple index out of range