The backup dirs are here:
backup_before_prepare_0 is the initial full backup
backup_before_prepare_1 is the 1st incremental backup
backup_before_prepare_2 is the 1st incremental backup
backup_0 is the prepared backup after all prepare were executed on it.
To reproduce the failure, restore backup_0 to the datadir and run the server on it. All server default options are sufficient.
If you have doubts about the correctness of prepare, you can run it on non-prepared backups.
mariabackup logs from all stages are attached with hopefully self-explanatory names (mbackup*.log).
I have no information yet whether it affects previous versions.