It turned out to be an issue with the fact that Mint is 1 LTS behind Ubuntu. So I had to use the http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_18.04/ instead of http://download.opensuse.org/repositories/isv:/ownCloud:/desktop/Ubuntu_19.04/
Once I realized that Mint was behind Ubuntu, I was able to update the repository list and it installed just fine. 