@pikachurus do you have set $g_allow_no_category = ON;?
If yes, does it work with version 2.26.3?

yes, $g_allow_no_category = ON;

I'll be able to check with 2.26.3 in a few days.

btw, in web interface i can change category to 'no category' with no problem.

The reported behavior is reproducible in 2.26.4 and the latest master branch with the request below. It appears to be a regression introduced by the fix for 0034683 - in 2.26.3 the same request does clear the category.

PATCH http://localhost/mantis/api/rest/issues/{{id}}
Authorization: {{token}}
Content-Type: application/json

{
  "category": {
    "id": 0
  }
}

Response code: 404 (Category '{"id":0}' not found.); Time: 71ms (71 ms); Content length: 92 bytes (92 B)

For the record, with MantisBT 2.26.4 and later it is currently possible to unset the category with the following payload. At this time I am not sure whether this is actually correct or even intentional, but at least for now you can use that as a workaround.

{
  "category": null
}

PR https://github.com/mantisbt/mantisbt/pull/2136