first check, if you have a filter set on this folder.
Select the folder that you moved your project to in the library inspector panel and look at the search fields in the browser and the library panel and look, if these search fields are showing "All items" or "Showing all". If not, clear them.
i am back working today and i am still seeing this behaviour. if i drag an album from a folder and put it into another folder i no longer see images/thumbnails whereas i did see them before.
i have looked just now in the filter entry area (thanks for the screenshot...) and i am not seeing anything listed in here. One says "All Items" and the other says "Unrated or Better".
i just moved another Album to an existing folder, lost all thumbnail images (nothing shown in list view either) and - then to test i dragged it back to the original folder.
now i am noticing that there is a cycling symbol at the bottom of the Aperture window....
is it possible that there is something going on here (too many images in the album) or some kind of CPU logjam that is causing me not to see images as described? it is seems like something i need to figure out and is sort of disconcerting at the moment.
maybe i the best way to test this by waiting for the cycling to stop i suppose - now that i have noticed this...
this is very bizarre. they are regular albums (i have not had a chance to use smart albums or stacks yet).
however, after having made a little lunch and read the paper for a bit - i am now back on the computer and all the images are now showing up.
it's very weird and a little disconcerting to say the least.
i even checked to see if Time Machine was running to make sure that this wasn't eating up CPU or something...
there are 161 images in the Album and it seems like this could cause big problems if it is not some kind of temporary issue...
- i am now back on the computer and all the images are now showing up.
This looks like Aperture need a lot of processing to simply move a regular library item. Have you had any crashes or force quits lately? Then I'd suggest to repair your Aperture Library.