I've written a quick guide on how to install the plugin and activate it for one domain, hope this is of some assistance to others: https://www.conetix.com.au/support/article/how-install-lets-encrypt-extension-plesk-125
If I've missed anything then please just let me know!
I've written up a quick workaround guide that we've pushed to any of our affected servers, hope this is of some use to those also experiencing the issue: https://www.conetix.com.au/support/article/roundcube-stops-working-plesk-12018-mu54