Issue

I was working on 10.4.1 with the Coveo upgrade and ran into this issue. Initially, I thought it was related to the Coveo issue, and the logs supported that.

Diagnosis

Looking at the Sitecore logs and the error on the page, it's clearly pointed out to Coveo.

Server Error in '/' Application.
This version of Coveo for Sitecore no longer supports the Sitecore Content Search API.
Description: An unhandled exception occurred.

Exception Details: Coveo.SearchProvider.Exceptions.DeprecatedContentSearchException: This version of Coveo for Sitecore no longer supports the Sitecore Content Search API.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[DeprecatedContentSearchException: This version of Coveo for Sitecore no longer supports the Sitecore Content Search API.]
Coveo.SearchProvider.ProviderIndex.CreateSearchContext(SearchSecurityOptions p_SearchSecurityOptions) +39
local.Web.Controllers.local.Components.NewsController.FindRecentNews(Int32 newsCount) in E:\agent_work\11\s\src\local.Web\Controllers\local\mysite\Components\NewsController.cs:45
local.Web.Controllers.local.mysite.Components.NewsController.GetRecentNews() in E:\agent_work\11\s\src\local.Web\Controllers\local\Components\NewsController.cs:31
lambda_method(Closure , ControllerBase , Object[] ) +87
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary2 parameters) +35 System.Web.Mvc.<>c__DisplayClass24_0.<InvokeActionMethodWithFilters>b__0() +80 System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func1 continuation) +466
System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +466
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +524

Solution

The solution was to fix the SOLR connection string. Once the connection string for SOLR was properly fixed, it worked as expected. I understand it's not relevant, but it took a long time to figure out the actual problem. The search indexes are all connected, whether it's SOLR, Coveo, or any other platform.

Hope this helps save you some time.