The following bug report has already been confirmed by Apple. Still I am posting it for the public so that it might help somebody who is getting the same problem.
The workaround is to override the readFromURL:error: method of UIDocument instead loadFromContents:ofType:error: since the problem lies inside NSFileWrapper. While being very convenient to use on smaller documents having too many items inside an NSFileWrapper will cause the problem described in this bug report.
Filed as rdar://13586175 and on Open Radar.
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