Try to set new created SSL certificate as default one (the same what button “make default” does) via command line:
# plesk bin certificate -u "default-2015" -default -admin SSL certificate 'default-2015' was successfully updated
But after this command nothing changed, the default certificate is still “default certificate”. The command doesn't work.
This issue is considered as Software Issue with id PPPM-2549.
This issue has been fixed in Plesk 12.0.18 MU#38. Please install all Plesk Updates.