Steps to Reproduce

1. Find an image with CMYK colour profile
2. Attach the image to a page
3. Edit the page
4. Inserting the image itself is fine
5. Go to Insert - Link > Attachments (or just Insert - Attachment) and you will see the error: "There was an error communicating with the server, please try again later.'

Workaround

• Open the image in image editing software and save it, specifying RGB as the colour profile if required. Usually just saving the file will convert the colour profile, on most simple image editing software.
• If you require the CMYK profile on the images, the workaround is to embed them directly in the page, by Insert - Image. This will display the image, and it works fine. It is only thumbnails that have the problem.
• Host/upload the images to somewhere else (e.g. Dropbox, image hosting site, external site, etc.), then insert the image link through Insert - Link > Web Link.

NB: Reproduced on the initial customer's space and on a clean instance of 4.2.11.