Hi,
I get the same probkem in Portal for ArcGIS 10.3
What ever I do (with the same or a different web Adaptor for the Portal), when I try to connect with my AD account, I get the message:
"Unable to sign you in to Portal for ArcGIS".
I followed all the instructions and repeated the process 3 times and still can't enter with my AD account or any other AD account.
Thanks a lot,
Miri