Error when changing a post privacy with the Facebook PHP SDK - php

Question

What I'm doing is creating a post with the Facebook PHP SDK(v4)(and the app is set to use the v2.3 API) with the privacy settings of value to SELF, then later on changing the value to CUSTOM, and allow to the ids of the friends that have accepted the app.
Until recently everything worked fine, and now out of the blue I'm getting this error(nothing changed on our part): "(#100) Object does not support message editing", which I couldn't locate in their documentation. Has anybody encountered this situation and this error?
Here is the code I'm using (like I said everything worked fine until recently)(if you need more details please let me know)
$fb_token = 'FB_TOKEN';
$FB_session = new FacebookSession($fb_token);
try {
$FB_session->validate();
} catch (FacebookRequestException $ex) {
// Session not valid, Graph API returned an exception with the reason.
//echo $ex->getMessage();
throw new RestException(501,'FB error: '. $ex->getMessage());
} catch (\Exception $ex) {
// Graph API returned info, but it may mismatch the current app or have expired.
//echo $ex->getMessage();
throw new RestException(501,'FB error: '. $ex->getMessage());
}
//make posting request to FB
if($FB_session) {
try {
$fb_array = array(
'privacy' => array(
'value' => 'CUSTOM',
'allow' => 'USER_ID'
)
);
$fb_req = new FacebookRequest(
$FB_session, 'POST', "/POST_ID", $fb_array
);
$response = $fb_req->execute()->getGraphObject();
//echo "Posted with id: " . $response->getProperty('id');
$fb_post_id = $response->getProperty('id');
} catch(FacebookRequestException $e) {
//echo "Exception occured, code: " . $e->getCode();
//echo " with message: " . $e->getMessage();
throw new RestException(501,'FB error: '. $e->getCode() .'-'. $e->getMessage());
}
}