I guess e.toString() is going to return the type of e as string, try using value instead
protected void naamValidator(object sender, ServerValidateEventArgs e)
{
bool check = true;
for (int i = 0; i > e.Value.Length; i++)
{
if (!(char.IsLetter(e.ToString()[i])))
{
check = false;
break;
}
}
e.IsValid = check;
}