Checking why Zendesk server rejects a JWT token

Hello,

I'm trying to integrate the Android Unified SDK with my mobile app and encountered a problem I cannot debug. I followed the documentation and set the JWT authentication in my initialization code (Zendesk.INSTANCE and Chat.INSTANCE). I also have a working dedicated JWT endpoint that returns valid JWT tokens (all 4 manatory fields present, valid signature, iat is a whole number). However, the Zendesk server does not accept the token and returns 401 Unauthorized to the SDK.

AFAIK the message flow looks like this:

                          POST                                              POST
        https://{domain}.zendesk.com/access/sdk/jwt                  {my-jwt-endpoint}
+------+  ------------------------------------->  +--------------+  ------------------>  +---------+
| SDK  |                                          |ZENDESK server|                       |my server|
+------+  <-------------------------------------  +--------------+  <------------------  +---------+
                    401 Unauthorized                               200 {"jwt": "<token>"}

I double-checked all conditions mentioned in https://developer.zendesk.com/documentation/classic-web-widget-sdks/support-sdk/working-with-the-support-sdk/building-a-dedicated-jwt-endpoint-for-the-support-sdk/  and I still don't know why the server returns 401. The server is basically a black box for me since I cannot view its logs. Any idea what I am doing wrong?

Additional info:

The token payload:

{
  "name": "Paweł Kunat",
  "email": "2000jvtuefoh3xaabigzhoeloee@customers.pawabank.com",
  "phone": "+441025222",
  "jti": "20Rzzg327I1DjtcVVTwPZGaIVCS",
  "iat": 1636022451
}

SDK logs (heavily truncated, couldn't find any other relevant entries):

11-04 11:50:35.982  1675  2036 I okhttp.OkHttpClient: --> POST https://pawaful.zendesk.com/api/v2/answer_bot/interaction (163-byte body)
11-04 11:50:35.989  1675  2036 I okhttp.OkHttpClient: --> POST https://pawaful.zendesk.com/access/sdk/jwt (313-byte body)
11-04 11:50:36.801  1675  2036 I okhttp.OkHttpClient: <-- 401 https://pawaful.zendesk.com/access/sdk/jwt (812ms, 37-byte body)
11-04 11:50:36.803  1675  2036 I okhttp.OkHttpClient: <-- 400 Response body was null, failed to auth user. https://pawaful.zendesk.com/api/v2/answer_bot/interaction (820ms, 2-byte body)

Init code:

Zendesk.INSTANCE.init(context, baseUrl, appId, oauthClientId);
Identity identity = new JwtIdentity(token);
Zendesk.INSTANCE.setIdentity(identity);
Support.INSTANCE.init(Zendesk.INSTANCE);
AnswerBot.INSTANCE.init(Zendesk.INSTANCE, Support.INSTANCE);
Chat.INSTANCE.init(context, chatAccountKey);
Chat.INSTANCE.setIdentity(jwtCompletion -> jwtCompletion.onTokenLoaded(token));

Versions:

messaging 5.2.0
support 5.0.3
answerbot 3.0.2
chat 3.3.0

minSdkVersion 16
targetSdkVersion 29

Subdomain: pawaful

I'm wrapping the SDK with React Native, but it works fine with anonymous identity. Only JWT causes problems.