sharepoint enterprise – Searching for anything besides ‘This Site’ returns error

We have SharePoint 2013 on-prem and I recently applied Windows security patches from October 2020. Since then, whenever we search on SharePoint for anything other than ‘this site’, this error is returned:

The base type ‘Microsoft.Office.Server.Search.Internal.UI.SearchResultsLayoutPage’ is not allowed for this page. The type Microsoft.Office.Server.Search.Internal.UI.SearchResultsLayoutPage, Microsoft.Office.Server.Search, Version=15.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c could not be found or it is not registered as safe.

I’ve found posts about others with this issue and tried their attempted fixes, notably updating the web.config file on all SharePoint servers to include the above, however it still is not working.

Does anybody have a successful resolution of this?