Getting vague "federal return not accepted" error

It was an incorrect Netfile code.

I am getting the same message 

I submitted by Netfile last year and brought my files forward from last year 

I have gone through forms and can't find an error 

I do have a T5008 form and input a summary but it seemed to be accepted 

I have not overridden anything 

Make sure you have the correct NETFILE Access Code (NAC). It's an 8-character alphanumeric code found in the upper-right hand corner of your Notice of Assessment (NOA).

 If it is correct, try filing your return without it, as it's optional.