F5 can't save to Storage from Background (while F6 and others can)

What exception is it throwing? If it's an ObjectStoreAccessException then it is because the device does not support 3.2.0 or later.

The API docs don't mention 3.2 and earlier background limitations, but it is in the developers guide. (https://developer.garmin.com/connect-iq/core-topics/persisting-data/)

Connect IQ 3.2 introduces the ability to access the Application.Storage module from background processes. The background process can 
modify storage using Application.Storage.setValue(), Application.Storage.deleteValue() and Application.Storage.clearValues(). When the 
storage is written from the background process, Application.onStorageChanged() callback will be invoked for the foreground process if 
the background and foreground process are active at the same time and vice-versa. The application would then have to reload data from 
storage to reflect updated information.

What exception is it throwing? If it's an ObjectStoreAccessException then it is because the device does not support 3.2.0 or later.

The API docs don't mention 3.2 and earlier background limitations, but it is in the developers guide. (https://developer.garmin.com/connect-iq/core-topics/persisting-data/)

Connect IQ 3.2 introduces the ability to access the Application.Storage module from background processes. The background process can 
modify storage using Application.Storage.setValue(), Application.Storage.deleteValue() and Application.Storage.clearValues(). When the 
storage is written from the background process, Application.onStorageChanged() callback will be invoked for the foreground process if 
the background and foreground process are active at the same time and vice-versa. The application would then have to reload data from 
storage to reflect updated information.

The exception is: 

Error: Unhandled Exception

Exception: Background processes cannot modify the object store

That will be it. Thanks for sharing.

Can you please update it to the API documentation?

Others might hit this exception on production, because you don't enable Background process testing and it's not clear. 

True, it is "now" in the developers guide, but that limitation should be clear in the Storage API documentation. These background context limitations are never clearly stated. There are others that I ran into via trial and error e.g. loading String resources in background context not being supported for older CiQ versions, I found no reference to that in the API docs. Thankfully I was clarified in the Forums.