Have just been working on a couple of New Mikrotik router installations. It would seem RouterOS 6.5 has a bug relating to the importation of SSL certificates.
It is easily fixed with a quick downgrade of RouterOS, install the certificate and then upgrade again.
#1: Download the 6.4 version of RouterOS -> (This is no longer listed on their website but I manipulated the url and it worked for me).
#2: Upload this file under the “Files” tab
#3: Open a new terminal – > /system package downgrade
#4: Import the certificate and install as you please, then upgrade the router back to 6.5 and you will still have the latest version (At the time of writing) installed with the certificate as needed.
Note: This issue seems to be fixed with the release of RouterOS 6.6