I have a standalone Lightroom 6 and just reinstalled it on a new computer after the other bricked. When I go to connect to old catalogues I now see several versions of each, such as Previews.Irdate (2) and several blank documents. When I click on them L6 asks to convert them to L6. They were already converted before. Any ideas?
I am not able to see any extension for the catalog file. But it seems you have multiple copies of catalog like Oblivion, Oblivion-2 and Oblivion-3.
Please try launching Oblivion-3(seems to be the updated catalog).
Let me know if this helps.