Invalid Length for a Base-64 char array

Hi all,
i'm using a datalist control to display all the videos and whenever a video thumbnail is clicked, a new window opens with the player.
i'm here sending encrypted query string to videos play window and there i'm decrypting it , i think there is no problem while encrypting and decrypting the query string
my problem is i'm having a data list with 20 items, when i click on 10-12 items continuously i can see the player window without any error, but after tht i'm getting this error
"Invalid Length for a Base-64 char array", I dont know what wrong is groing there..
can anyone please help me sorting this out as i spended 2 whole days on this ..


here is my encryption, decryption code:


public class videos { private stat ...