I can't sign in to my Portal or Online Login from ArcGIS Pro as well. Though I'm not getting the error that you are. I'm not getting any error at all! Either it just says "Not signed in" or Pro closes down, when I try to lo in to my Portal.
I have configured my Portal with ArcGIS Pro licenses and assigned a license to myself, and I can see them from within my Portal, but apparently not use them.
Any thoughts? Need an answer because my trial is ending and want to work on program please!