Fatal error: Call to a member function setDefaultValueCallback() on null in **

Hello,

I have a Drupal version 8.2.4 installation using Aegan 8.x-1.0 theme, up and running, hosted on inmotionhosting.com  for about one year by now. Recently I noticed the Drupal Configuration tab bringing this error:

“Fatal error: Call to a member function setDefaultValueCallback() on null in /home/aaa/public_html/aaa.aaa.com/core/modules/comment/src/Entity/Comment.php on line 221” 

That means now I can not access the “…/admin/config/” link of my website. I am not able to make any configuration to my site anymore. The worst  case is that I am not able to post new blog articles, resulting in the same error message.

I am not experienced in Drupal programming, except that I can configure and install in any server infrastructure. As a solution I tried to upgrade to newest version of Drupal, resulting in to another mess. I don’t know  how to resolve this problem and would like to get your kind help. Thanks.

Drupal version: 


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