I just realized that the version you are using, 1.3.99.3, does not yet contain
the patch that I mentioned.

Change akonadisyncsource.h as shown in comment #1 and recompile. In other
words, it should look like:

     AkonadiTaskSource(const SyncSourceParams &params)
        : AkonadiSyncSource("application/x-vnd.akonadi.calendar.todo", params)

Because of the incorrect type, the search for items of the right type never
succeeds, leading to the empty list. I was thrown off track because I thought
the incorrect type also (and only) prevented listing the task collections (that
was the original error report), and that worked for you.