Delete your prefs. If problem persists, run repair. If that doesn't work, run rebuild. Direction are on the Apple Aperture Trouble-shooting page.
Afaik, Split View shows one Browser and one Viewer. Strikes me as odd that the Browser browser is not showing the same thing as the Split View browser. You may simply have a UI problem.
In any case, the steps above should help. Let us know what you find out.
I'm a bit reluctant to run a overall repair as it may corrupt other Projects.
You still have your backup, don't you? So you could revert to your saved state, if need be. After you set up a new user on a nrew machine I'd at least epair the permissions with the first aid tools.
How did you restore the Aperture library from the backup? Was it done with as part of a setup with Migration Assistant? Or did you restore the library separately from Time Machine?
Does the project in question have managed originals or referenced originals? You may need to reconnect referenced originals after migration.
You cannot repair just a single project, but if there is an inconsistency between Browser and Viewer, regenerate the thumbnails for the images in the project, also the previews.
Select the images in the browser and use the command "Photos > Generate Thumbnail". When that is done, hold down the ⌥-key and use the command "Photos > Generate Preview".
Are your thumbnails now consistent with the Viewer image?
Strikes me as odd that the Browser browser is not showing the same thing as the Split View browser.
Perhaps there is a difference in Quick Preview mode, Kirby? I have no time to experiment right now.
It would really help to know, how JD8001 migrated the user account and Aperture library, and if the user account name is still the same, to see, if there is a settiings problem or the library needs repairing or rebuilding.
Thanks for this advice, however the problem is only with one Project out of many Projects. All the other Projects are OK. So, is there any way I can do a repair or rebuild on the one corrupted project? I'm a bit reluctant to run a overall repair as it may corrupt other Projects.
If this happens again, you might have success by simply creating a new project within Aperture and dragging the files from the corrupted project to the new one. This is basically like rebuilding the project. I've had it solve seemingly insolvable problems before.
Excallent, your suggestion to "Photos > Generate Thumbnail" worked. The problem has been solved and all the thumbnails are now consistent with the Viewer image.
To answer your questions: The Project in question only has originals. I migrated the Aperature data from a backup using Time Machine with the same user account and Aperature library as on the backup. The user account name was also the same as the process was a full download of all info, software, date files etc from the backup disk in setting up a new MacBookAir laptop. My old MacBookAir laptop was stolen so the backup saved my day and I was able to seamlessly (aside from the Aperature issue we have now solved) move everything on the old laptop to the new laptop with the backup info. Lesson? Always backup, backup, backup.