Hello,
I was able to resolve this. An old .dll from a different plugin's directory made it's way in to the 4.0 project directory. It has been there since 3.8, but for some reason created no errors in 3.9. Regardless I have removed it and everything is alright now. Thank you for the help!
When I try to run nopCommerce in Visual Studio. I get the following error:
System.Exception
HResult=0x80131500
Message=Plugin 'Nop Ajax Filters'. Method 'GetActionFilterAttribute' in type 'SevenSpikes.Nop.Plugins.AjaxFilters.ActionFilters.FiltersActionFilterFactory' from assembly 'SevenSpikes.Nop.Plugins.AjaxFilters, Version=3.8.248.21753, Culture=neutral, PublicKeyToken=null' does not have an implementation.
I have re-downloaded the plugin from here and tried again in case I had a wrong version or something and got the same result. Any ideas?