"Unable to reach app. Please try again." Error on Message Extension

I'm using a Bot-Framework bot that was built on a self-hosted n8n server rather than through the SDK.

Whenever I try to open a Dialog, regardless of the way I open it, I receive an "Unable to reach app. Please try again." error.

The Teams logs show the following under my user:

2025-08-10T08:03:47.047Z Err	CID[1bcdb590-4b7f-470d-80ab-c9ac4978eee8] TaskModuleForMessageExtensionContainer: [useFetchTaskForMessageExtensionEntryPoint] Failed to invoke task for threadModality: Unthreaded with error
2025-08-10T08:03:47.047Z Err	CID[1bcdb590-4b7f-470d-80ab-c9ac4978eee8] TaskModuleForMessageExtensionContainer: TaskModule:Unthreaded invoke failed:  Error code: 1008 message: <BotError>
2025-08-10T08:03:47.046Z Err	resolverUncaughtErrorBoundary__doNotAssignToGraphQLTeam: Uncaught error in a GraphQL resolver for operation "taskForMessageExtension", at field selection path "taskMessageExtension" - {"message":"Error code: 1008 message: <BotError>","locations":[],"path":["taskMessageExtension"]}
2025-08-10T08:03:47.046Z Err	resolverUncaughtErrorBoundary__doNotAssignToGraphQLTeam: Uncaught error in a GraphQL resolver for operation "taskForMessageExtension", at field selection path "taskMessageExtension"

Weirdly enough the request does reach my bot which processes it as intended and responds to the webhook with an adaptive card. I also tried responding with the example from here without any processing whatsoever and it didn't work either.

My thoughts are that either:

1. I am not supposed to send the Adaptive Card as a response to the webhook but to some endpoint.
2. I don't have necessary permissions (currently the only permissions my bot has are the RSC ChannelMessage.Read.Group and the Graph ChannelMessage.Read.All), but I don't think that's likely considering the "processing" part of the bot's response only includes variable mapping to the card without any use of any API.
3. It's somehow related to the authorization of the response itself.

Some additional details:

• Bot ID is 21b3b297-d020-48b2-8e29-88901d919f57
• It has other Teams-related functionality involving reading and writing messages on channels which works perfectly
• I tried to invoke it specifically from the channels it's been installed on
• Average Request Latency according to the Azure Metrics is about 300ms