I'm encountering a similar issue to what was reported here: I get the same error whenever I attempt to save a second instant target on a single InstantTracker. This error is also present in the "Instant Tracking - Save Instant Target" sample; saving a target, pressing the reset button and then attempting to save another target will fail with the same error.
I'm using Unity version 2019.2.4f1, SDK version 8.9.0. I'm encountering this error in both the Unity built-in player, and when building and testing on Android.
There does seem to be one workaround - recreating the tracker immediately after saving:
Tracker = Instantiate(Tracker)
But this causes a new error message, "No Instant Tracking running when saving scene!", to be reported (even though saving produces valid targets that can be loaded normally).
Am I misunderstanding the sample, or is this a bug?
We've identified the issue and it is scheduled to be fixed in the next update. Thank you for reporting the problem.