Bill,
I just can't remember where it was. I would look inside the Catalog itself (on a Mac, use Show Package Contents to see what is inside - the Finder will pretend that it's a file, but it's really a folder.) To be specific, right-click on the Catalog file and there should be a popup menu with "Show Package Contents" as one of the choices. Then you will see folders and files with names like Adjustments, Originals, "Capture One Catalog.cocatalogdb", etc. I THINK that's where the write lock file was... but look for anything suspicious.
Have you tried simply rebooting? Works more often than it should...
Best,
Matt
PS. I also think I had to specify an output folder before it would process the first time. Again, I forget how I did that. The joys of aging.