For anyone that might stumble upon this these days:
If you also get an error validating your ArcGIS server connection -
1.) Your machines may have been desynchronized during a restart. According to the docs, it will only attempt to synchronize the Portal and ArcGIS servers once, then it's up to you to synchronize them manually. See: Synchronize With Site—ArcGIS REST APIs | ArcGIS Developers.
In case the link is broken later, see the description at the bottom of this post.
2.) Before you unregister your web adaptor, try the tool in the above link. It only takes a few minutes to run.
3.) You can access the ArcGIS Server Administrator Directory using the following URL: https://<machine name>:6443/arcgis/admin. I was able to access this even when the other server URLs were inaccessible.
Hope it helps someone!
Description of the Synchronize with Site tool:
"On occasion, one or more machines in a server site might be unavailable due to network issues or because they are down (intentionally or unintentionally). Once these machines become available again, they will need to synchronize with the site to pick up any changes made to the site during that downtime. This is done automatically by the site, but it is only a one-time attempt. If there are any issues with this synchronizing effort, a SEVERE message is logged.
This operation allows administrators to manually synchronize specific machines with the site. Synchronizing a machine with the site will reconfigure the machine and redeploy all services. This will take a few minutes. During this time, all administrative operations on the site will be blocked."