ISAPI Filter ‘C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll’ could not be loaded due to a configuration problem

Opened IIS Manager and selected the IIS web server node:

In the middle pane, browse to the IIS section and select ISAPI Filters:

Expand the columns as required, looking for an Executable path with \\ in it, then click Edit:

From the above screenshot, change the \\ to \ so the path reads:

C:\Windows\Microsoft.NET\Framework\v4.0.30319\aspnet_filter.dll

Just keep in mind that the folders Framework and Framework64 or the .NET version may change depending on your implementation.

After clicking OK, all my testing started to work again and the 500 Internal Server errors went away.