Problem with a encrypted Cookie

Hi Guys!
I have a problem with cookies.
The app is in Visual Studio, C# and MVC 3.
My app store some info and settings in a cookie. Even that the info is not that important, I encrypt the content.
For encryption I used machine key.
The situation is this:
After the app finished to load (and after the cookie is created), if I hit refresh, the app fail to reload again.
While debugging, no of the breaks gets a hit, no matter is a putted in the first line of code. However, if I remove the encryption and store the values directly (plain text), everything works flawless.
I verified the encryption method, and I’m sure that its works properly.
The cookie itself is about 755 B.
Any Idea what can be causing this??
Thanks! Edgar.
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