Problem Importing Aperture Library

When I import an Aperture Library into Capture One 10, A triangular symbol with an exclamation point appears to the right of the library folder and all of its sub-folders in the Folders section of the Library side bar. All the images in this Aperture Library inside Capture One appear fine and all are online. The image files are stored on an external hard drive, not inside the Aperture Library. I can access and adjust the images and I can process them and see and use the processed results outside Capture One.

What does this warning symbol mean? How can I find out what is wrong so I can correct the problem? Or, is it the case that any imported Aperture Library will show this warning symbol and I should ignore it? But if that is the case, why have it there at all?

I have had no problems using Capture One with files imported from memory cards or from locations on my internal or external drives. Nor have I had problems (warning triangle) when I import images that were once part of my Aperture libraries using the Import dialogue. I only have problems when I import using the Import Aperture Library command.

I could just import all my old photos that were part of Aperture using the Import dialogue, but this would mean I would loose the adjustments I made in Aperture (as I understand it). The advantage of using the Import Aperture Library command is the retention of these adjustments (or most of them).

So, have I got this process all wrong? Should I be doing something significantly different? I can't find anything in the videos, blogs, or forums that addresses this. If there is, I'd really appreciate a link - or whatever help you can offer. Thanks.

Using an iMac, late 2015, 27inch 5k, OS 10.12.4 Sierra, Lacie 1TB and 3TB external drives.

Regards, Stephen