This problem started with the update to version 3.5 (now on 3.5.1).
Generally I can import new raw images without problems. Files are copied to the library, previews created. When I start editing (or perhaps even just viewing) the images, at some point an image will suddenly turn black with the exclamation mark saying 'unsupported image format. When this happes, going back to a previously correctly shown image will also turn black.
Closing and re-opening Aperture often helps. Once I open the image and open the hud, Aperture retries reading the RAW file and the image comes back. Occasionally Aperture will persist in claiming that the RAW file is unsupported and there is nothing I can do but restart the iMac completely.
Sometimes it will not show this exclamation mark, but it will distort the image. Wrong orientation, in all shades of pink or just some lines of pixels showing. This is more difficult to correct, as Aperture 'thinks' that it has already read the image. Which it has, but incorrectly.
Again occasionally the thumbnail of the image will flicker between the exclamation mark and the correct preview. 'Show activity' shows that Aperture is looping and constantly trying to generate a preview for whatever reason.
Displaying the HUD sometimes helps to get Aperture to read the RAW file correctly, other times it slows down completely and fades into a a complete hangup that I can do nothing about other then forcefully quit Aperture.
I use RAW files from two cameras, the Pentax K5 (.DNG) and Sony A7 (.ARW). Both cameras are supported. Then I use NIK software with the latest release and using TIFF as file type. I created a new Aperture library i.e. without older files edited with NIK (.PSD I beleive) 'contaminating' the preview creation process - to no avail. The issue happens every Aperture session and will result in me having to quit and restart Aperture again and again and again.
Unfortunately I was not able to discover a scenario or that the issue is limited to a particular file type. I've worked for years with the .DNG's from the Pentax K5 without any issues, now I get the same problems with them as the .ARW's and .TIFF's.
What can I do?