Look for the Response.Redirect() calls in the page previous to the form you are getting the error. Just make sure that any harcoded URL for the form supplied to the Response.Redirect() call is correct.
EDIT..
Also, look for If you are navigating to a form which is in a different directory than the current one. For example:-
Response.Redirect("../SomeForm.aspx");
will redirect to an ASPX page just one level up of the current one. Each pair of dots implies one directory level up.
In thses cases, I think, you are better off using ~ (tilde) character which will always take the path from the root. So the mistakes happening because of incorrect number of dots can be minimised.Try something like this:-
Response.Redirect( this.ResolveUrl("~/Myfolder/SomeForm.aspx") );
More details for the ~ and ResolveUrl here.
This can also happen If you have window.location calls in javascript if you assign an incorrect URL to the same.