For a specific user, I have an invalid postback when he attempts to upload a file from a data entry form as well as submitting some data entry fields which are related to it. I see that this user have this error on a frequent basis. This is something I cannot simulate. I am trying to know what is causing this.

I have this message:

"Invalid postback or callback argument. Event validation is enabled...'

All my forms are defined the same. But, this is the only form which has an upload button among the other fields. And, this is the only place in the application I have this error. But, it seems to only happen to a few users only.

What should I look for?