Cannot launch Lightroom Classic 14.0 with a large catalog that was fine with version 13 builds.

I don't know the cause but I cannot open the new version of Lightoom with my existing (large-1m) catalog. The update occured as I was in the process of doing a clean update to Windows 11 from Windows 10 on my desktop. I have a full back up of the catalog that worked fine with the previous version. Version 14.0

insists on updating it, but after the update it will not work with the catalog. Just get  'An error prevented lightroom classic from opening this catalog'. No error message when performing the conversion, just on re-launching.

I've tried the suggested Adobe solution of deleting the CURRENT file and forcing Lightroom to re-start. Makes no difference. Have also tried uninstalling and re-installing v14.0, again, same issue. This is critical to me as I was half way through processing a major event. Plus in the last year I've moved from largely using Capture 1 to Lightroom Classic. A move that now seems an error. I'm a professional photographer and rely on the Adobe's software I'm subscribing to working.

Is it an option to go back to the a version 13 build?