ODP.NET error Unable to find the Requested .Net Framework Data Provider

For us it was 32 vs. 64 bit process.

The server is 64 bit. The ODP.NET (Oracle Client) installed is also 64 bit. Our application compiled with the Target platform "Any CPU" and "Prefer 32-bit" flag SET:

http://grab.by/v5ki

was running as 32 bit process. Once recompiled with the flag un-checked everything started to work.

I got the same error when opening the page via IIS, 64bit Win7.

My solution is:

Go to IIS manager --> Application Pool --> Advanced Settings --> Enable 32-Bit Applications.