Marc,
I had similar problems when building an all-in-one development VM.
You're right to address the SSL issues first to ensure you're not getting any requests rejected due to certificate issues. For an all-in-one I use the following approach:
1. In IIS either create a Self Signed Certificate or a Domain Certificate (see Create a Domain Server Certificate in IIS 7 on TechNet) and then use this to create SSL bindings on your default site.
2. Export this certificate from IIS to your desktop.
3. If it's a self-signed certificate then install it on the local machine into the 'Trusted Root Certificate Authorities'
4. Copy the certificate to any machine that will access the Dev system and install as per Step 3.
5. Open the ArcGIS Server Rest Admin and navigate to the machine and navigate to 'home > machines > machineName > sslcertificates > importExistingServerCertificate' to install the certificate saved to your desktop in 3.
6. Step back in the crumb trail to the machine and edit the machine properties. Update the SSL certificate to the certificate imported above. This then ensures that the AGS internal web server is using the same certificate as IIS and will be recognised by your client(s).
7. Switch to the Portal for ArcGIS admin page (https://<server>/<webadaptername>/portaladmin) login using a Portal Administrator's credentials and select Security > SSL Certificates. Repeat the import Existing Server Certificate as performed in step 5.
8. Once imported, use the 'update' operation to update the Portal web server to use the certificate you've just imported.
9. Perhaps a bit of overkill, but I prefer to now restart the VM to ensure that the services come back up with the right certificates.
With a domain certificate you may need to check to ensure that the CA's root certificate has been installed into your Trusted Root CA store to ensure that your domain certificate can be validated.
I'd done all this and had the error you originally highlighted. I found that Martin's pointer to add the Portal instance as a data store then solved my problems. (Thanks Martin!)
Tony