Why do I get a 500 error when I try to get a list of group photos?

Question

Classmates

Created a query like this:

https://api.ok.ru/fb.do
?aid=group_main
&application_key=*app_key*
&fields=photo.id
&format=json
&gid=58713497075965
&method=photos.getPhotos
&sig=*sig*
&access_token=*token*

In response I get:

{
error_code:2
error_msg:SERVICE : Service is temporary unavailable.
error_data:null
}

Maybe I'm using something wrong.
My task is to get photos from the community wall. Tell me what to use for this?