Thanks!
I don't think that I am having any connection issues to the Dropbox servers. I have full access to other files and folders in Dropbox, just not these specific files from 2004.
The Firefox I tested with was a brand new install, no extensions or plug-ins. Same results as for Chrome, Chrome incognito, Chrome Beta, Edge, and Edge Dev.
I get the same grey icons on Mac and Windows, and on multiple computers.
I also tried on my phone, using only cellular data and thus bypassing my router/firewall and any desktop-related antivirus, firewall, security, proxy or VPN - same result.