Why is there an error "error.groups.common.notmember" when posting a media topic to my group?

Question

Classmates

Good day!
Recently received the rights (GROUP_CONTENT) to post a media topic to the game group.
When posting a media topic via the mediatopic.post method, an error occurs:
"REQUEST : Invalid request : error.groups.common.notmember"
mediatopic.post with parameters:
type = GROUP_THEME
gid = XXXXXXXXXXXXX
attachment =

{
    "media": [
        {
            "type": "text",
            "text": "This is a text of a new topic"
        }
    ]
}

Tried to remove the gid - swears at his absence.
What could be the problem? And what does this error mean?
Thank you!

Answer 1

The user on whose behalf they are trying to make a post is not a member of this group.
Or the application has EncodingSeed!=0 set. In this case, before sending a request for a media topic post, you must perform the "groupID xor EncodingSeed" operation and substitute the result of the operation in the request instead of groupID

Answer 2

Faced the same problem as the topikstarter.
And how to find out the encodingseed of the application? I don't see this option in the app settings.

Answer 3

It worked, thanks!
There was a second case with EncodingSeed!=0:
"it is necessary to perform the operation 'groupID xor EncodingSeed' and substitute the result of the operation in the request instead of groupID"
EncodingSeed is the id of the game creator in OK.