You may also see this error message displayed in the views on a page (the page itself loads ok):
Error: Unable to validate data source credentials. Please make sure to provide the right credentials for your data source.
This second error is because the views in NV are trying to connect to the pollers web service, and are failing due to the same permission issue that causes the error 500.19 issue on the master.
This is caused by insufficient permissions being in place in IIS.
NFA, ADA and Netvoyant.
Navigate to C:\windows\system32\inetsrv\config\ and open the file called applicationhost.config.
Search (Ctrl+F) the file for the entry for overrideModeDefault, and change that setting to overrideModeDefault="Allow".
Open a command prompt on the server, and go to C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727
Once there, issue the following command:
Then reset IIS by running the command "iisreset".
NOTE: For NetVoyant, these steps need to be performed on the NV Master and all the pollers. For SuperAgent 9.0, this also needs to be done on the collectors.
After those changes, refresh the web and the errors should clear.