And even now that the cause of the problems is clear, developers can't do much more than advise users to wait. For example, after a week of troubleshooting, Realmac Software, developer of the Clear to-do-list apps for iOS and Mac, today published a support article stating, "It appears that iCloud is currently having problems with the Documents and Data feature that Clear (and all other iOS apps) use to store your data...We've filed this bug with Apple (#15048686) and are awaiting further advice."
Unfortunately, before discovering that the issue was being caused by a larger iCloud issue, some developers recommended standard app-troubleshooting procedures, including deleting apps and reinstalling them, and even asking users to create new data and attempt to sync with iCloud to "jump start" a sync. In theory, the original data is still in iCloud and will be restored to these apps once Apple fixes the issue, but there's always a chance that new data created for troubleshooting reasons will overwrite what's in iCloud, depending on how each app handles sync and that app's sync status at the time the problem started.
What can you do if you've been afflicted with this issue? Other than contacting Apple Support to let them know that yours is one of the problematic accounts, not much other than to wait for the eventual fix.
Fixes in progress
Speaking of which, Apple support personnel have indicated -- in phone, chat, and email exchanges with affected users -- that Apple is aware of the problem and is working on a solution (though Apple's System Status webpage still doesn't indicate any iCloud issues). On Apple's support forums, many users have reposted messages they've received from various support personnel.
For example, user "KLasc" received an email from iWork support stating, "Please note that Apple is aware of the issue and we are investigating this at the moment." And user "Kathryn Jensen" received a call from Apple Corporate Executive Relations and was told that (in her words), "Executives at Apple are aware of the iCloud ubiquity account locked issue, and that the system status page isn't reporting any issues."
Indeed, some users on Apple's Support Communities forums have reported that their accounts are again working normally, though at the moment, for every such report, many more new complaints are appearing.
At the time of publication, Apple had not yet responded to a Macworld request for comment. We'll update this article with any such response.