Since upgrading to the first beta of iOS 8.1, I started noticing that a few apps were randomly hanging or crashing at launch. I thought that the problem would be fixed for the public release of iOS 8.1, but soon after updating to the stable version, it occurred again.
I then thought that a clean install of iOS 8.1 would be the solution, but both my iPhone 6 and iPad Air 2 on a clean install of iOS 8.1 (not restored from a backup) are still experiencing the iCloud hanging/crashing bug. For the past week, apps like MindNode, Twitterrific, Pixelmator, and (before the latest update) Drafts have been hanging or crashing at launch on my devices, forcing me to force-quit them, reboot (with a hard reset), or manually copy data because iCloud wasn't syncing. Each app was tested with existing document libraries as well as an empty database.
iOS 8.1 shipped with a serious bug affecting apps that use iCloud. If you have seen apps randomly hang, especially on launch, it’s likely you are seeing the effects of this issue. Many apps use iCloud for a variety of data storage needs, so even apps that you were not aware were using iCloud may exhibit issues related to this bug.
As he notes, there's little third-party developers can do to fix the issue:
There is little developers can do to avoid this problem, so if you are a user seeing this issue, please be patient waiting on a fix and do not blame your friendly neighborhood app developer for this problem. Especially, do not blame them in an App Store review.
Greg managed to ship a workaround with Drafts 4.0.3, but that's not a solution to the underlying problem of iCloud and iOS 8.1. Based on my informal polling on Twitter, the bug doesn't affect all iCloud accounts but it is common, and even when an iOS device is affected the issue is intermittent and only partially “fixed” by rebooting or logging out of iCloud (read: it'll happen again after a few hours or days). In short, there doesn't seem to be a solution for affected accounts for now.
Hopefully the issue will be fixed soon by Apple, as it is a serious problem that makes apps with iCloud features unreliable and essentially unusable. If you're developer, read Greg's post for more technical details.