Inaccessible install due to SSL
For SSL issues, login as the root user on the SynergyCP web host and run:
/scp/bin/scp-exec php_s php artisan ssl:remove
This will disable SSL on the application, you can then access the application via regular HTTP.
Failed SSL Certificate generation
If you're getting errors about failing to generate an SSL certificate, first make sure all 3 domains on the Settings > HTTP tab are pointed to the same IP. Proxies/CDNs such as Cloudflare could cause issues as well, so try turning any of those off.
Also, Let's Encrypt has a rate limit of 5 attempts per week for the same domain name combination. Even if you have only attempted once and got a failure for some reason, the system may automatically retry the validation process. In this case, you could try changing the Admin URL of your application so that the certificate is no longer a duplicate and then re-attempt the process. Make sure to point the Admin domain to the correct IP before updating the value in SynergyCP.