I've upgraded the Joplin version to Joplin 2.6.9 and see the same error when checking for updates and checking the synchronization configuration.
A similar issue as this one appeared when I started using the AWS CLI, but I resolved that problem by exporting my local certificates to a ".pem" file and referencing the file to the AWS CLI with the command export AWS_CA_BUNDLE=/etc/ssl/certs/Certificates.pem. Any way I could reference this same file with the Joplin client?