Unrecognized attribute 'targetFramework'. Note that attribute names are case-sensitive

The problem could be:

• the Application Pool for your site is configured for .NET Framework Version = v2.0.XXXXX
• .NET 4 isn't installed on your server.

See also

• Unrecognized attribute 'targetFramework' ...
• How to add ASP.NET 4.0 ...

... which helped me fix a similar issue.

You'll likely need to re-register ASP.Net version 4 with IIS.

Navigate to your .net framework 4.0 folder, and run aspnet_regiis.exe -i

For example, my .net framework 4.0 folder (Server 2k8 R2 x64) was located at: C:\Windows\Microsoft.NET\Framework64\v4.0.30319

Run the following command to fix this problem.

Start --> Run:

%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe –i

If you get an error about ADMIN rights you need to do the following.

1. CTRL+SHIFT+ENTER from the RUN menu will run as ADMIN
2. START --> PROGRAMS --> ACCESSORIES --> Right-click on command prompt and "run as administrator"