Why does FormsAuthentication.Decrypt crashes when moving to another Server?

The reason why the encrypted data couldn't be decrypted on another machine is because the validation key in the original machine's machine.config is set to AutoGenerate. This means that the generated keys are unique to each machine. A better solution will be to specify the validation key, so you can decrypt it across any machine with the same validation key.

For more details on solutioon refer following article :

Microsoft Knowledge Base Article - 313091

Microsoft Knowledge Base Article - 312906