Hi Lara,
went to the procedure a couple of days ago without the problem you described.
Just to get things in a row:
In my understanding there are (at least) three types of Portal-Logins:
1. initial admin
2. main portal admin
3. portal user
I succeded to update portal by taking the main portal admin account at this update dialogue. What login type did you use?
Another idea would be that ESRI changed the password requirements in between the versions. Can you still login into the 10.3.1 portal (or portaladmin) to change the main-portal-admin-password to a strong password as Jayanta suggested and retry updating portal afterwards? (I suggest you have to go via port 7443 as you certainly deleted the webadaptor...)
Good luck!
Simon