When you were on ColdFusion 10, were you serving content out of ColdFusion's web root (\ColdFusion10\cfusion\wwwroot\...)? That appears to be the case based on the error above. If you are using IIS, you should not be serving sites out of ColdFusion's web root. If you have to, use IIS's web root (C:\inetpub\wwwroot), although a best practice is to put your web-accessible files on a completely different drive from your OS (and ColdFusion should be on it's own separate drive). So here is what I do (based on the various ColdFusion lockdown guides:
C: - OS Files
D: - ColdFusion installation (D:\ColdFusion10\...)
E: - Web Sites (E:\WebSites\site1, E:\WebSites\site2, etc.)
The web.config is an IIS configuration file (and the HTTP 500.19 error is coming from IIS). This indicates your IIS site configuration is probably at fault. You might want to strip things down and rebuild the website per the CF11 lockdown guide, and rebuild the site in IIS accordingly.
I'm not even clear how the content was being served out of CF's web root. I thought it was setup as IIS. An Adobe developer removed CF10 and installed CF11 on my machine. I'm having to fix the mess so that I can continue working. Sigh.
Thank you for your help!
It probably evolved over time. My guess is that development was started using ColdFusion's built-in server, then later the site was connected to IIS without moving the files to a different location.