OK, my starting point is that LR can happily contain 400000+ photos, and it's a mistake to create a 1 star plus catalogue as you suggest. Instead, I'd recommend you create a master catalogue of all your photos, which you can then query however you want - 2 star, or a certain keyword, or with "xyz" in the title etc.
If you are intent on going that way, you could try a slight change from what you've done so far. Instead of trying to import an entire catalogue and then delete no-star pictures, take the 2014 and 2012 catalogues and select only the 1 star photos. Then do an Export as Catalog for these items, and import the 2014-1star and 2012-1star into your new catalogue. That may get you past the problem images which I suspect are the problem.
If that doesn't work, or if you follow my initial advice, you're going to need to identify the image where the failure occurs. You may have a corrupt image file, and replacing it from your backup may be sufficient to resolve the problem. Alternatively, it may be a corrupt record in the catalogue, and in this case it may be sufficient to remove the image from the catalogue and import it again. You might try selecting everything in the 2014 catalogue and seeing if you can do an Export as Catalog, creating a clean 2014, and deleting 2014's previews folder might also get rid of a potential problem. Something else is to try the Export as Catalog for images 1-40000, then a second Export as Catalog for images 40002-50k.
So a few things you'll need to try, rather than a magic bullet, and a recommendation to change the underlying direction.
Great! Thanks John for the general advice and I will try your suggestions and let you know a mark the result accordingly.
Hi John! I have been trying really hard but unfortunately it still didn't work!
1. I tried exporting the needed photos an new Catalog and that worked. But when I try to import them again from my Master *+ Catalog I get the same error again!! Any other ideas? I guess now thinking about it, I should may be create a new catalog and try to import from that and see if it works?
2. So that means that I cant even merge all my catalogs into one as you suggested - but on that point, are you 100% sure later on when I solve this problem, it is a good idea to have 500-700k photos all in one catalog?
I actually now tried creating a new catalog and tried to import the 2014 *1 catalog and the same error happened!!
Any advice on solving the problem would be highly appreciated....? As well as your feedback on having 1 catalog of 500-700k photos. Thanks.
What about your Lightroom versions? I think I once saw the error when I had created a catalogue on one computer which had a newer version of LR and moved it to another computer where I did the Import from Catalog.
You done all the other things like deleting the previews folders?
What about by 40002 suggestion? Let's say 40000 exactly are imported and it fails at 40001. So can you create a 2014*1 items 40002-50000 catalogue, and does that import? Or delete 40001 and see if it now works.
As you say, until you solve this problem you can't merge into a single catalogue.