I spent a quite of time on this problem. I have re-entered everything directly again in the form view.
Not 100% sure if this is solution but I finally got through it and Netfiled:
If the T3 has a Source Country (ie not blank), the both first & second box of 52, 53 must be filled in (Foreign capital gain).
If the T3 has a blank Source Country (ie Canada sourced), only the first box of 52, 53 needs to be filled in
