Transfer failure, with rc = 3

[sbendt]

I have a board connected to a PC by USB - the board is configured for CDC / ACM, and shows up as a functional port.

Using the mcumgr-client application, I can connect to the board, list the images, reset etc. When uploading I do get a failure:
...
19:01:19 [DEBUG] (1) mcumgr_client::transfer: expected length: 16
19:01:19 [DEBUG] (1) mcumgr_client::transfer: result string: ABADAAAGAAGjAb9icmMD/yT8
19:01:19 [DEBUG] (1) mcumgr_client::transfer: response header: NmpHdr { op: WriteRsp, flags: 0, len: 6, group: Image, seq: 163, id: 1 }
19:01:19 [DEBUG] (1) mcumgr_client::transfer: cbor: bf62726303ff
19:01:19 [DEBUG] (1) mcumgr_client::image: response_body: {
"rc": 3
}
19:01:19 [ERROR] Error: rc = 3

The board is flashed with the Zephyr in-tree MCU manager.

I am not sure how to troubleshot the rc = 3 upload failure, or any resource that defines the rc values.

Same result with and without the patch mentioned in the readme file.

Any insight on this?