I'm a new ("about a week old") user of xcp-ng and Xen Orchestra and observed the same problem as described in this forum post.
The older (20.04.6 LTS) Linux installation I moved out of ESXi just for testing purposes is the one I'm testing on for now, but this problem seems to be easily reproduced on the newer versions as well (and I will test it within a few days).
From the backup I create, I can list the partitions in the backup, but am only able to select files from the boot partition (1.42GB), the two other shows the red exclamation mark.
The behaviour is the same with both XO from source and the just activated full trial of XOA (for testing the file-level restore).
Anyone solved this yet ?