If you have the Acesef extension installed in your managed site, then you will get a Can't Connect message when you try to communicate with it. To solve it, just disable the Acesef route for com_securitycheckpro:
Also check if the site is online. If the site is offline the Control Center won’t be able to reach remote installation (which is located in frontend).
A few versions ago I added some extra security improvements to avoid certain attacks during communication between control center and remote servers. One of them was adding verification to SSL certificates through the curl directive CURLOPT_SSL_VERIFYPEER. Basically this makes a comprobation of every SSL certificate; some sites return the following message: SSL certificate problem: unable to get local issuer certificate. This means your php configuration doesn't have a valid cacert.pem file configured (see this link ).