iCloud Drive is not loading on iOS 13
Solution 1:
If I had a device that was doing this, I would use my developer account to load a specific profile that enables debug logs and see if I could parse if this is a certificate error / date error / server side condition.
- https://developer.apple.com/bug-reporting/profiles-and-logs/?platform=ios
If you have Xcode, you could look at the normal console logs as well to see if anything pops up, but these logs are incredibly noisy and could be a college level course for 3 months to teach someone how to use them to isolate an issue. I don't mean to make it impossible, but a lot of effort might be needed to triage and fix this directly. Even with these logs, I would consider opening a case with Apple Dev or regular support as they have the engineers that designed it and the source code. We only get the log file output.
The good news, is anyone can call Apple Support since iCloud is supported and they will go through the usual items - sign out of iCloud (which is the opposite of the above - enabling a profile is a surgeon's scalpel - signing out is like knocking down the room) and then sign back in. Also, wiping the device and starting over - signing in to Files first might be a reasonable thing to try if you have a good backup. It will take 10 minutes to test and then worst case, you confirm it's broken and erase / restore your backup and then engage with debugging yourself or with AppleCare.
Good luck - something is definitely messed up if this persists for days and everything else works.
Solution 2:
A restart of the iOS device solved the problem for me.