Problems displaying site content

Sergey_Lelekov - A big thank you for your post.  While the problem resolved by anandpeculiar also solved my issue, the problem first manifested in a different way in our SharePoint Farm. 

Please forgive the length of the reply but I'm hoping it may further help others encountering the same problem.

In our environment the problem first appeared with a variety of Display Template related error messages similar to these two samples:

"The display template had an error. You can correct it by fixing the template or by changing the display template used in either the Web Part properties or Result Types. Cannot read property 'indexOf' of undefined (CoreRender: ~sitecollection/_catalogs/masterpage/Display Templates/Search/Item_CommonItem_Body.js)"

"The display template had an error. You can correct it by fixing the template or by changing the display template used in either the Web Part properties or Result Types. No included language files were loaded for locale (en-us) or the site's default language (en-us) for string ID (rf_RefinementAriaLabel_More). Please make sure you include the string IDs required by this Web Part in a language file that your Display Template references in the Site Collection Master Page Gallery. (LoadResource: )"

I eventually stumbled on to your post because I happened to notice the same occurrence of blank site management pages in our SharePoint Farm, too. anandpeculiar's advice to install KB5001974 was the fix we needed.

Further digging led me to this https://blog.stefan-gossner.com/2021/07/13/july-2021-cu-for-sharepoint-server-2019-is-available-for-download/ announcing the July 2021 CU for SharePoint Server 2019. The blog states "Important: It is required to install both fixes [KB5001974 & KB5001975] (language dependent and independent) to fully patch a SharePoint server. This applies also to servers which do not have language packs installed. The reason is that each SharePoint installation includes a language dependent component together with a language independent component. If additional language packs are added later (only) the language dependent fix has to be applied again."

In our environment, the update service only applied KB5001975 but ignored the critical fact that KB5001974 should have been installed too!

Hope this helps others!!