> Where does it fail?

It fails on different points within the build. In my custom layer configuration it fails when building the u-boot-script recipe.

IMHO the problem appears due to running those recipes in parallel (for each multiconfig) and  some races appear

when mounting/unmounting the WORKDIR into the rootfs.

The next problem may appear if created packages of one MACHINE will overwritten by the same package of another MACHINE.

Packages must contain a machine specific suffix in order to fix that.

> Which Isar revision?

I ran those builds with the current next branch.