Personally, this problem occurred while requesting the endpoint insights :
https://graph.facebook.com/v2.9/<ad_id>/insights?fields=account_id,account_name,action_values,actions,ad_id,ad_name,adset_id,adset_name,app_store_clicks,buying_type,call_to_action_clicks,campaign_id,campaign_name,canvas_avg_view_percent,canvas_avg_view_time,canvas_component_avg_pct_view,clicks,cost_per_10_sec_video_view,cost_per_action_type,cost_per_estimated_ad_recallers,cost_per_inline_link_click,cost_per_inline_post_engagement,cost_per_total_action,cost_per_unique_action_type,cost_per_unique_click,cost_per_unique_inline_link_click,cpc,cpm,cpp,ctr,date_start,date_stop,deeplink_clicks,estimated_ad_recall_rate,estimated_ad_recallers,frequency,impressions,impressions_dummy,....
Finally, find out that the impressions_dummy field was causing the problem, so by removing it from the request, the problem was resolved.