After the following steps, the system partition is not marked as active, causing an unbootable system:
1. Restart the installer on a working reactos system.
2. In the Select Partition screen, choose the existing partition.
3. In the Format Partitions screen, press ESC to return to the previous screen.
4. Delete the existing Partition.
5. (Optional) Press P to create a new primary partition
6. Press enter to install in the Unpartitioned space or the new primary partition which has just been created and finish the installation.
Result: After the reboot there is an error message "No active partition found"
The attached patch fixes this, but I'm not sure this is sufficient in more complex configurations.