Message text:

After Effects can’t continue: Damaged item.

Message interpretation:

An unspecified item appears to be damaged.

Possible causes:

This error more frequently appeared with the availability of intel Nehalem Xeon based Mac Pro systems and seems to be related to using Hyperthreading and multiprocessing. In those cases the file I/O overhead seems to jumble up the data streams.

Resolution or workaround:

You may try to turn off Hyperthreading as explained in this error code listing. Generally, though, it is sufficient to turn of the Render Multiple Frames Simultanously feature in your After Effects preferences. even with the feature enabled, restarting the program should at least temporarily fix the issue.