@Arnie & @Viret -
Thank you for finding this. You are correct, this field should be optional for portals. It is only required for Email opt-outs (a new feature which BTW is pretty cool - read about it here http://www.methodintegration.com/method/kb.aspx?folder=methodcrm&id=165).
While it is not required, it is still beneficial to set the contact record id as well as the activity record id so that you can track who clicked on the link and when.
For now, just set the contact record id to 0, and it will let you save.
On the next update this will be resolved - we were able to duplicate it the issue and resolve it in the labs. We're expecting a "May version" publish late next week.