IIS 8.5 HttpModule load only under CLR v2

I'm testing a simple http module on IIS 8.5 + W2012R2. The module works well on IIS 7 + 2008R2 but does not load on the new machine configuration. After changing different IIS settings, I managed to make the module load again by changing the following 2 settings:

1) On the Server Node, change .NET framework version from V4 to V2

2) On the DefaultAppPool, change .NET CLR from V4 to V2

Once IIS is restarted, the module can be loaded again. The module is written in C# VS Express 2012 with Target .net framework 2.0.

But even if I recompile the module to target at .net framework 4.0, and restore the setting of 1) & 2) above to their default values (CLR 4.0), the module still doesn't load.

Are there any method to troubleshoot this ? Thanks.