Regarding issue 500 service unavailability in drupal 7 - ajax

Question

Hope you Guys are doing good. I have 1 issue with my project functionality. I have 1 form in that form we have few fields and in last we have submit and download button. when we will click on submit and download button that time we will this error.
POST https://admin.default.com/manage_doc_download_popup/ajax/2091/674?token=... 500 (Service unavailable (with message)) Response Text: [{"command":"settings","settings":{"basePath":"\/","pathPrefix":"","ajaxPageState":{"theme":"capstore","theme_token":"gcu_X3w1KcbG7Lq6TmlXVMOupzbkdowwVgbpz_a-Xlo","js":{"sites\/all\/modules\/contrib\/ctools\/js\/ajax-responder.js":1}},"lightbox2":{"rtl":0,"file_path":"\/(\\w\\w\/)public:\/","default_image":"\/sites\/all\/modules\/contrib\/lightbox2\/images\/brokenimage.jpg","border_size":10,"font_color":"000","box_color":"fff","top_position":"","overlay_opacity":"0.8","overlay_color":"000","disable_close_click":1,"resize_sequence":0,"resize_speed":400,"fade_in_speed":400,"slide_down_speed":600,"use_alt_layout":1,"disable_resize":0,"disable_zoom":0,"force_show_nav":1,"show_caption":0,"loop_items":0,"node_link_text":"View Image Details","node_link_target":0,"image_count":"Image !current of !total","video_count":"Video !current of !total","page_count":"Page !current of !total","lite_press_x_close":"press \u003Ca href=\u0022#\u0022 onclick=\u0022hideLightbox(); return FALSE;\u0022\u003E\u003Ckbd\u003Ex\u003C\/kbd\u003E\u003C\/a\u003E to close","download_link_text":"Download Original","enable_login":false,"enable_contact":false,"keys_close":"c x 27","keys_previous":"p 37","keys_next":"n 39","keys_zoom":"z","keys_play_pause":"32","display_image_size":"","image_node_sizes":"()","trigger_lightbox_classes":"","trigger_lightbox_group_classes":"","trigger_slideshow_classes":"","trigger_lightframe_classes":"","trigger_lightframe_group_classes":"","custom_class_handler":0,"custom_trigger_classes":"","disable_for_gallery_lists":true,"disable_for_acidfree_gallery_lists":true,"enable_acidfree_videos":true,"slideshow_interval":5000,"slideshow_automatic_start":true,"slideshow_automatic_exit":true,"show_play_pause":true,"pause_on_next_click":false,"pause_on_previous_click":true,"loop_slides":false,"iframe_width":900,"iframe_height":520,"iframe_border":0,"enable_video":1,"useragent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit\/537.36 (KHTML, like Gecko) Chrome\/86.0.4240.193 Safari\/537.36","flvPlayer":"\/sites\/default\/files\/DCXCOE_DX_sv.mp4","flvFlashvars":""},"overlay":{"paths":{"admin":"file\/*\/downloads\nfile\/add\nfile\/add\/*\nfile\/*\/edit\nfile\/*\/usage\nfile\/*\/delete\nnode\/*\/edit\nnode\/*\/delete\nnode\/*\/revisions\nnode\/*\/revisions\/*\/revert\nnode\/*\/revisions\/*\/delete\nnode\/add\nnode\/add\/*\noverlay\/dismiss-message\nuser\/*\/shortcuts\nadmin\nadmin\/*\nbatch\ntaxonomy\/term\/*\/edit\nuser\/*\/cancel\nuser\/*\/edit\nuser\/*\/edit\/*\nnode\/*\/moderation\nnode\/*\/moderation\/*\/unpublish\nnode\/*\/moderation\/*\/change-state\/*\nnode\/*\/moderation\/view\nnode\/*\/moderation\/diff\nnode\/*\/moderation\/diff\/list\nnode\/*\/moderation\/diff\/view\nnode\/*\/moderation\/diff\/view\/*\/*","non_admin":"admin\/structure\/block\/demo\/*\nadmin\/reports\/status\/php"},"pathPrefixes":[],"ajaxCallback":"overlay-ajax"},"bootstrap":{"anchorsFix":"0","anchorsSmoothScrolling":"0","formHasError":1,"popoverEnabled":1,"popoverOptions":{"animation":1,"html":0,"placement":"right","selector":"","trigger":"click","triggerAutoclose":1,"title":"","content":"","delay":0,"container":"body"},"tooltipEnabled":1,"tooltipOptions":{"animation":1,"html":0,"placement":"auto left","selector":"","trigger":"hover focus","delay":0,"container":"body"}}},"merge":true},{"command":"insert","method":"prepend","selector":"head","data":"\u003Cscript src=\u0022https:\/\/admin.default.com\/sites\/all\/modules\/contrib\/ctools\/js\/ajax-responder.js?qjdm9a\u0022\u003E\u003C\/script\u003E\n","settings":null},{"command":"invoke","selector":".comment-form","method":"show","arguments":[]},{"command":"invoke","selector":".comment-section .fivestar-widget","method":"removeClass","arguments":["pointer"]},{"command":"css","selector":".comment-section .fivestar-widget div.on:last a","argument":{"width":"100%"}},{"command":"redirect","url":"https:\/\/admin.default.com\/file\/2091\/download?token=OEJk7avE","delay":0,"new_window":false},{"command":"modal_dismiss"}]
and will get error in this line in JS file
xhr.send( ( s.hasContent && s.data ) || null );
So guys if you have any idea about this please help me.And I'm new in this project so I don't know, how to start.. Please help guys. I have attached few images related to issue that will help to understand what the issue is.[Error 1] [Error 2]
Please help guys..!!!