We use this app on multiple devices and also on PC (web site). About Amazon bought Appps running already (since the beginning of December 14) with the version 4 (automatic update) that the Google Play gates at that time still with V3. There you get the V4 only as a beta tester and there is no information on the incompatibility of V3 to V4. Long we had to try until we got our apps synced back (with the loss of all our current shopping list).
Something like this should not happen eigendlich and otherwise there would have been 5 stars.