After performing an OS upgrade from 2012 R2 -> 2019 or similar, you may get the following error when trying access the website afterwards:
Server Error in ‘/’ Application. Configuration Error Description: An error occurred during the processing of a configuration file required to service this request. Please review the specific error details below and modify your configuration file appropriately.
Parser Error Message: The ‘targetFramework’ attribute in the element of the Web.config file is used only to target version 4.0 and later of the .NET Framework (for example, ”). The ‘targetFramework’ attribute currently references a version that is later than the installed version of the .NET Framework. Specify a valid target version of the .NET Framework, or install the required version of the .NET Framework.
This can happen because the version of .NET that came with the OS changed, so that the prerequisite .NET framework for EIOBoard is no longer met by the Server. You can fix this by downloading the .NET 4.8 framework here:
If any other prerequisites are missing, you can find the whole list for the server prerequisites at this Knowledgebase article: