OmniFocus 2.9.1 fixes bugs:

• Archive — Fixed a problem where archiving immediately after migrating to v2.9’s new database format would fail.
• Attachment List — The “Added” column for parent rows (the attachments themselves) now correctly displays the date an attachment was added.
• Sync — Fixed a problem where OmniFocus didn’t recognize that an error returned by NGINX (that OmniFocus’ data/ directory hadn’t been created yet) was recoverable. OmniFocus should once again be able to sync to NGINX WebDAV servers without a workaround.
• Crash — Fixed a crash that was new in v2.9.
• Crash — Fixed a crash with attachment preview generation that was the eventual result of a transient bug during our 2.9 test period.
• Sync Preferences — OmniFocus now warns you about possible data loss when disabling sync.
• Sync Errors — Updated the error message that appears when OmniFocus is unable to overwrite one of its own files on the server. This is a problem currently affecting customers syncing using Jianguoyun (坚果云).
• Localizations — Updated localizations.

OmniFocus 2.9 introduces a new attachment storage and sync system. To migrate to the new system you’ll need to be running OmniFocus 2.19 or later on all of your iOS devices and OmniFocus 2.9 or later on all of your Macs. Once all of your syncing devices are running a version that supports the new system, you’ll be prompted to migrate your database.

• Attachments — Starting with v2.19 (iOS) and 2.9 (Mac), OmniFocus stores attachments separately from the items that contain them, resulting in improved performance and storage efficiency. Once all of your syncing devices are running a version that supports the new attachments, you’ll be prompted to migrate your database.
• Attachment List — The Attachment List window now groups multiple items that reference the same file and provides buttons to easily go to an item.
• Completed Perspective — This perspective no longer shows a group of “Not yet completed” items.
• Repeats — Prevented a crash when importing an item with a repeat rule that OmniFocus doesn’t understand.
• Software Update — Fixed a problem where you might receive “CFErrorDomainLaunchd error 4” when trying to update OmniFocus from within the app.
• Sync Preferences — Fixed a problem where the Unregister button for sync clients was active with no selection in the client list (and unregistered the first client in the list).
• Sync — OmniFocus now handles the DAV 207 multi-status responses returned by NGINX in its default configuration with the nginx-extras APT package.
• Device List — Added the OmniFocus version of the listed device as a tooltip both in the Device List in Preferences and during the database migration flow.
• Documentation — Updated the in-app Help.
• Encryption — We now offer to immediately re-upload and re-encrypt your database when you change your encryption passphrase.
• Future Database Formats — Improved the error message you’ll see when OmniFocus tries to open a database from a later version.
• Localizations — Updated Localizations.