Should be fixed with commit 329ab80e62e87867cbdd25da4ba75fb5117da050 (disable strict parsing instead of adding the lang attribute, because we don’t use it and there might be other unknown properties)
Your contacts.xml is empty (see https://github.com/rfc2822/davdroid/blob/master/res/xml/contacts.xml)
Consider using my proposed one to enable all fields (like birthday, which is not enabled right now).
Proposed for davdroid: https://gist.github.com/dschuermann/6954672
Also see this example from Google: http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android-apps/4.0.4_r1.2/packages/apps/Contacts/tests/res/xml/test_basic_contacts.xml?av=f
Added contacts.xml and birthday support with commit 6ee010905e172164b865c0e9c0576d92d1ae7906. However, I can’t test it because I’ve only got Android 4.1- devices and they show no birthday field.
I tried syncing a birthday field, and it works as expected.