The option to enable iPhone sync doesn't appear under the Feature tab in Preferences in version 1.3.99.*. Besides that, when I tried to downgrade to version 1.3.27, the older version cannot open the files saved by the newer version.
1.3.99 is not newer
1.3.99 is the testing version for new features before they are ready for release. The newest version is 1.3.27, which is not a downgrade. 1.3.99 is not for normal use.
Make sure you have your file backed up, then edit it in a task editor and change the version in your .tsk file to 1.3.27 and it will open.
I've changed the release version number in both the tsk file and the ini file, and it still refuses to open. The last like of the error reports: File "taskcoachlib/persistence/xml/reader.pyo" line 251, in _parse_task_node KeyError:'status'. I did not find 'status' anywhere within my tsk file.
we'll try to help, but do not use 1.3.99 as your main system, it is only for testing, it can break things!
It looks like you installed a build from the feature-storage-refactoring branch, which is a highly experimental branch, not even guaranteed to launch... Your task file is now missing a property I removed in this branch. Try the latest "bugfix" build (from http://www.fraca7.net/TaskCoach-packages/latest_bugfixes.py upper section), it will give a default value to this attribute. You'll probably end up with duplicate tasks on your iPhone though, you should reinstall the iPhone version first.