Hi Colinw,
Thanks for sharing the details, would you mind trying the steps mentioned in this video after 1.17sec: - Cloud Documents for XD Mobile v02 - YouTube and share the results.
Moving the discussion to Get Started community.
I'd rather not make an exception for XD files by organizing them separate from the rest of my projects and files. Enabling Creative Cloud Sync allowed me to successfully download my XD files and open them.
Once I transfer everything back to my computer, I will likely be discontinuing my use of Adobe's cloud storage service. These kinds of errors without apparent cause or solution have created more problems for my workflow than they have made it more efficient or convenient. I appreciate Adobe's forum staff efforts on this issue and will mark this as resolved.