Problem importing iPhoto library in Aperture
Not sure if this will solve your problem but it might be a good starting point.

Try rebuilding the iPhoto library. You can do this by holding down the Command and Option keys while opening iPhoto. You will then need to choose the rebuild options. I think it would probably pay to select all of them. iPhoto will then work away for a long time and rebuild the library.

There is an Apple support web page that details this process further. I would normally say that you should keep a backup before making any changes like this but I guess you still have the copy of the library on the old iMac

If the rebuild doesn't help then I'm not really sure what you can do.