I've seen this problem before when the Web Adaptor had the machine name listed in Web Adaptor URL rather than the alias used by the end users.
1. login to portal admin : https://yourmachinename.domain:7443/arcgis/portaladmin
2. click system > web adaptors > you will find your webadaptor name. click the web adaptor name, you will get the url like this https://yourmachinename:7443/arcgis/portaladmin/system/webadaptor/cc1dbd26-03ac0497c-69cbd8fc6cc8 (this is random id, maybe different in your machine)
3. in the browser append /edit to the end of the URL like this : https://yourmachinename:7443/arcgis/portaladmin/system/webadaptor/cc1dbd26-03ac0497c-69cbd8fc6cc8/edit then push enter.
4. edit Web adaptor URL, change with your domain. by default the url is machinename > click web adaptor. the portal is restarted, wait for the portal to restart before proceeding.