James, I'm also seeing the error that Kevin sees using iOS with Okta and Enterprise logins to our Portal for ArcGIS 10.5.1. I followed your instructions and when I login Survey123 passes me to a browser as you said it would then when I log in, I'm receving the following error "Could not access any Portal Machines. Please contact your system Admistrator." The URL I'm taken to is https://dot.esri.com/portal/sharing/rest/oauth2/saml/signin. I've never seen this error logging into our portal from any other app. Any ideas?
Eric J. Rodenberg | Solution Engineer Transportation Practice
Esri | 7775 Walton Pkwy, Suite 270 | New Albany, Ohio 43054-8202 | USA
T 614-933-8698, ext. 5503 | erodenberg@esri.com | esri.com