I am Using Sigma Osclass Theme 1.3.1
Site become Sticky or Freezed at the time of Ad posting for about a min. Then after sometime when It moves to payment option then it unfreez and start working. It is happening at every ad posting. Below is the Debug Log. But found nothing suspicious and helpful in that error log. Please help. 

[20-Feb-2021 09:30:03 Asia/Kolkata] PHP Notice:  Undefined index: tags in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/tags/item_form.php on line 5
[20-Feb-2021 09:30:43 Asia/Kolkata] PHP Notice:  Undefined index: tags in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/tags/item_form.php on line 5
[20-Feb-2021 09:41:08 Asia/Kolkata] PHP Notice:  Undefined index: i_repeat in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/osclass_pay/model/ModelOSP.php on line 911
[20-Feb-2021 09:46:39 Asia/Kolkata] PHP Notice:  Undefined index: HTTP_USER_AGENT in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/voting/index.php on line 84
[20-Feb-2021 09:46:40 Asia/Kolkata] PHP Notice:  Undefined index: HTTP_USER_AGENT in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/voting/index.php on line 84
[20-Feb-2021 09:46:40 Asia/Kolkata] PHP Notice:  Undefined index: HTTP_USER_AGENT in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/voting/index.php on line 84
[20-Feb-2021 09:47:31 Asia/Kolkata] PHP Notice:  Undefined index: HTTP_USER_AGENT in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/voting/index.php on line 84
[20-Feb-2021 09:56:35 Asia/Kolkata] PHP Notice:  Undefined index: HTTP_USER_AGENT in /home/scorpffz/public_html/postfreeonline.com/oc-content/plugins/voting/index.php on line 84

Try to disable all plugins and if working ok enable plugins one by one. You should also check browser console.

Try to disable all plugins and if working ok enable plugins one by one. You should also check browser console.

I already tried by disable all plug-ins still the problem was not solved.

Search on forums...
And you will find its caused by wrong configuration of mail server.