Empty responses from 3x-ui API when connecting clients via /panel/api/inbounds/addClient endpoint in version 2.6.2.

[theorangecolorv2]

Describe the bug

When using the API to add clients via endpoint /panel/api/inbounds/addClient, the system periodically returns empty responses. This prevents the creation of new clients for some users, although for others the requests are successful. The problem is non-permanent: for example, for user_id 1173379288, all three attempts fail with an empty response, whereas for user_id 5636013803, the request passes on the first attempt.

How to repeat the problem?

Use the add_client method with the inbound_id and email specified (in the format of the user_id string).

Send a POST request to /panel/api/inbounds/addClient with payload:

{
"uuid": "<generated_uuid>",
"email": "<user_id>",
"totalGB": 0,
"expiryTime": 0,
"enable": true
}

Observe an empty response from the server for some requests even after three attempts with delays

Expected action

Expected behavior:
The API returns a JSON response:

{
"success": true,
"obj": {...}
}

with data about the added client.

Received action

Actual behavior:
For some users, the API is a simple response (without a body) that causes responses in the code and the inability to access the website. The problem does not affect all users.

Logs:

Unexpected request for user_id 1173379288:

2025-07-14 19:13:21,358 - xui_api - INFO - 1/3 Attempt to add a client for email 1173379288.
2025-07-14 19:13:21,366 - xui_api - WARNING - Attempt 1 failed: X-UI returned an empty response.
2025-07-14 19:13:21.367 - xui_api - INFO - Retry in 0.5 seconds...
2025-07-14 19:13:21.868 - xui_api - INFO - Attempt 2/3 to add a client for email 1173379288.
2025-07-14 19:13:21.873 - xui_api - WARNING - Attempt 2 failed: X-UI returned an empty response.
2025-07-14 19:13:21.874 - xui_api - INFO - Retry after 1.0 seconds...
2025-07-14 19:13:22.875 - xui_api -INFO - Attempt 3/3 add client for email 1173379288.
2025-07-14 19:13:22.879 - xui_api - WARNING - Attempt 3 failed: X-UI returned an empty response.
2025-07-14 19:13:22.879 - xui_api - ERROR - Failed all 3 attempts for email 1173379288. I give up.

A convenient query for user_id 5636013803:

2025-07-14 17:17:57.951 - xui_api - INFO - 1/3 Attempt to add client for email 5636013803.
2025-07-14 17:17:57.974 - xui_api - INFO - Successfully ADDED client 1167cdc6-00f5-49e1-abb5-8d705c52827a for email 5636013803 on the first attempt.

3x-ui Version

2.6.2

Xray-core Version

25.6.8

Checklist

• This bug report is written entirely in English.
• This bug report is new and no one has reported it before me.

[nickshiro]

In my case, the endpoint addClient returns 404 error

[theorangecolorv2]

In my case, the endpoint addClient returns 404 error

think you have a different problem than mine

[nickshiro]

In my case, the endpoint addClient returns 404 error

think you have a different problem than mine

no, POST only on addClient/ enpoint return 404 without null data, including through the panel itself