Hi Aitziber,
i've written a little bit of JavaScript code, which opens the application in an external window without any portal frames.
So instead of accessing the iView/WD4A when clicking on the respective portal navigation, you call a simple HTML-file from KM) with the mentioned JavaScript. Then a new tab with the WD4A application will be opened without any error messages.
I know, this is just a workaround, but we were not able to find a correct Portal-side solution. Unfortunately, SAP Support wasn't able up to find the problem so far. The only thing i can say is that in my case, the problem is definitely caused by the portal an not by the backend systems.
Regards
Michael