Are you mac or win? If win I would totally un-install C 7.1x. Phase has some instructions on their website on a complete uninstall process. If you are a catalog user, I believe you will lose that, but there might be a way to back it up. They also have instructions for a mac uninstall. If you are running a mac, you may need to re-name your Capture One 6 to something like Phil's Capture One 6, as both versions will load in the mac app's folder as Capture One and this can cause some confusion. I had to do this on my mac when I installed 7 or I got a similar error, 7 would never load but 6 would fine.
You can also just go back to a system restore point prior to the install of Capture 7.1.x., restore the system and re-install.
Phase always seems to like having older versions removed, but I still have 6 installed on my main machine with 7.1.1 and no problems.
The Catalog is independent of the application so you will not affect any catalog by uninstalling Capture One!!!
I think I should make that very clear.
Phil - Its hard to give suggestions without knowing the full config. Have you opened a support case?