I believed this is an IIS related issue:
Orion is looking on this file: C:\inetpub\SolarWinds\Orion\NetPerfMon\Alerts.aspx
Have you tried to rebuild the IIS?
You may want to try the following below:
1. Stop IIS service
2. Go to C:\inetpub\Solarwinds
3. Rename the Solarwinds Folder to Solarwinds_Old
Then run the Configuration Wizard to rebuild the Website. Just select 3 option and let the Configuration Wizard finish.
I have done that, and in both the old and new folder, the file is missing....
I've run the Configuration Wizard countless times......
Since you have installed WPM before, probably there's still a WPM alert enabled on your Advanced Alert manager. Can you please check?
If you do, please have them disabled or better yet - delete them since the variables used are not available anymore.
Let me know how it goes.