post comment on facebook error: An unexpected error has occurred. Please retry your request later

Question

I get an error when I post a comment in my post:

code Koala:

Koala.config.api_version = "v2.0"
@graph = Koala::Facebook::API.new(access_token)
comment = @graph.put_comment(params[:comment][:id_ref_facebook], params[:comment][:content])

code FB_graph:

post = FbGraph::Post.new(params[:comment][:id_ref_facebook])
bool = post.comment!(
 :access_token => access_token,
 :message => params[:comment][:content]
)

I get this error:

message: An unexpected error has occurred. Please retry your request later. [HTTP 500]

The comment is posted in facebook, but the unique ID of the comment is not returned.