Transcode error: cannot create the ama sample mapper to play this clip

6 cards of ProResHQ from a Pix or similar recorder.

1920x1080p, 59.94.

AMA into an empty 1920x1080p 59.94 project. Transcode in background to DNxHR LB for inclusion in ongoing project already transcoded to DNxHR LB.

Every task errors out with cannot create the AMA sample mapper to play this clip

Did a few tests not using the background queue. Worked fine.

Anyone?