Our SharePoint Server is build in one box. (SQL and SharePoint)
We encountered 500 internal server error on all the other site collections that was created. Even newly created Web Applications are having the same issue (we created a new web application to test if we can have an up and running SharePoint site - but no. Problem is still there).
All Application Pools are running (started) with no login issue.
Looking at the ULS, we have encountered the following Critical and High Levels logs:
Unknown SQL Exception 1346 occurred. Additional error information from SQL Server is included below. A transport-level error has occurred when sending the request to the server
SqlCommand: 'proc_FetchDocForHttpGet' CommandType: StoredProcedure CommandTimedout: 0 Parameter: '@RETURN_VALUE' Type: Int Size: 0...
Error occurred right after a reboot of the server.