Forms Authentication failed - The ticket supplied has expired.

Hi all,
My server keeps going down randomly on a daily basis with the message "Server Application Unavailable " and a message to check the log.
The log shows the following error:
Event code: 4005
Event message: Forms authentication failed for the request. Reason: The ticket supplied has expired.

Can anyone please tell me how to fix this problem?
My Code:
protected void LoginAction_Click(object sender, EventArgs e)
    {
        Page.Validate();
        if (!Pag ...