Some customers of Kolab Now have reported connection issues using native applications on iOS 11.
The issue seems related to SSL certificate trust problems, possibly related to the use of intermediate certificate authority certificates. References from across the web indicate this issue had allegedly been to Apple as early as July, but it seems the problem is not yet resolved in iOS 11, released on Tuesday, September 19th, 2017.
We advise customers that use the native iOS applications to synchronize mail, calendars, address books and notes, such as the ActiveSync, IMAP, CalDAV and CardDAV account types, to postpone upgrading to iOS 11 if possible, until after this issue can be resolved. There’s currently no prognoses when this might happen, and Kolab Now staff is researching possible options to resolve the connection issues on our end.
Downgrading from iOS 11 to iOS 10.3.3 is possible, but requires iTunes, a wired connection and will cause you to lose all files stored on the device. An iOS 11 backup can not be restored to an iOS 10.3.3 device.
UPDATE (2017/09/20 13:50): ActiveSync appears to work without problems.
UPDATE (2017/09/20 13:59): IMAP works, but users may be unable to select individual folders (INBOX can be read in “All Inboxes”). The IMAP folders appear “grayed out”.
UPDATE (2017/09/20 14:08): CalDAV, CardDAV confirmed to work without problems. The issue really seems limited to only the native IMAP client. Customers can use an IMAP app from the App Store, or use ActiveSync.
UPDATE (2017/09/29 15:21): Apple released IOS 11.0.1 with a number of fixes. The issue at hand is however not fixed in this new release.
UPDATE (2017/10/03 09:52): Disabling IMAP IDLE on the server side does not resolve the issue.
UPDATE (2017/10/09 15:31): Thanks to Greg, we have the following two new references: https://discussions.apple.com/thread/8089268 and https://discussions.apple.com/thread/8074826. Neither have received a response from Apple so far.
UPDATE (2017/10/10 18:32): Upgrading to iOS 11.0.2 does not resolve the problem. So far, we are displeased with Apple’s silence.
UPDATE (2017/10/20 12:17): Upgrading to iOS 11.0.3 does not resolve the problem.
UPDATE (2017/11/07 08:14): A workaround has been applied that appears to resolve the problem.