When installing an SSL certificate via the Acquia Cloud UI, you may, on occasion run into an error that states.

An unknown error occurred. Please try your request again. If the problem persists, contact Acquia Support.

The number one cause for this error that affects Acquia Cloud Professional customers is that the ability to have two active certificates is unsupported at this time on Acquia Cloud. It is likely that you are attempting to install a new certificate while the old one is still in place.

The way to resolve this issue is to deactivate the old SSL, then add and activate the new one.