proxmox-backup-client - Error: Is a directory (os error 21)

[msantosn]

I am doing a PoC of PBS to backup files from a server.

I have setup PBS 4.1.0 and from that end seems fine, but I installed a client on Ubuntu 22.04, I have tried these two:
- http://download.proxmox.com/debian/...oxmox-backup-client-static_4.0.16-1_amd64.deb (4.0.1)
- https://cloud-pbs.com/wp-content/uploads/2025/06/proxmox-backup-client (3.4.1)

And I get the same error every time, I am able to backup, but I cannot see the catalog or restore. Getting:

storing login ticket failed: $XDG_RUNTIME_DIR must be set
Error: Is a directory (os error 21)

This is how I backup:

# export PBS_REPOSITORY=10.201.3.126:vxnl01a
# export PBS_PASSWORD=XXXX <I have also tried with a file and being prompted for the password>
# proxmox-backup-client version
storing login ticket failed: $XDG_RUNTIME_DIR must be set
client version: 4.0.16
server version: 4.1.0
# proxmox-backup-client backup etc.pxar:/etc
Starting backup: host/THUNDER/2026-01-01T11:10:33Z
Client name: THUNDER
Starting backup protocol: Thu Jan  1 12:10:33 2026
storing login ticket failed: $XDG_RUNTIME_DIR must be set
Downloading previous manifest (Thu Jan  1 12:10:21 2026)
Upload directory '/etc' to '10.201.3.126:vxnl01a' as etc.pxar.didx
Error downloading .didx from previous manifest: Is a directory (os error 21)
etc.pxar: had to backup 2.644 MiB of 2.644 MiB (compressed 646.203 KiB) in 0.59 s (average 4.461 MiB/s)
Uploaded backup catalog (39.392 KiB)
Duration: 1.26s
End Time: Thu Jan  1 12:10:34 2026

The error about $XDG_RUNTIME_DIR throws me off, but it seems it is normal.

And this where I can see the snapshot is there, but I cannot do anything else.

# proxmox-backup-client snapshot list
storing login ticket failed: $XDG_RUNTIME_DIR must be set
┌───────────────────────────────────┬───────────┬────────────────────────────────────────────────────────────────┐
│ snapshot                          │      size │ files                                                          │
╞═══════════════════════════════════╪═══════════╪════════════════════════════════════════════════════════════════╡
│ host/THUNDER/2025-12-31T15:43:33Z │ 2.683 MiB │ catalog.pcat1 etc.pxar index.json                              │
├───────────────────────────────────┼───────────┼────────────────────────────────────────────────────────────────┤
│ host/THUNDER/2025-12-31T16:18:02Z │ 2.683 MiB │ catalog.pcat1 etc.pxar index.json                              │
├───────────────────────────────────┼───────────┼────────────────────────────────────────────────────────────────┤
│ host/THUNDER/2026-01-01T11:10:21Z │ 2.683 MiB │ catalog.pcat1 etc.pxar index.json                              │
├───────────────────────────────────┼───────────┼────────────────────────────────────────────────────────────────┤
│ host/THUNDER/2026-01-01T11:10:33Z │ 2.683 MiB │ catalog.pcat1 etc.pxar index.json                              │
├───────────────────────────────────┼───────────┼────────────────────────────────────────────────────────────────┤
# proxmox-backup-client catalog dump host/THUNDER/2026-01-01T11:10:33Z
storing login ticket failed: $XDG_RUNTIME_DIR must be set
Error: Is a directory (os error 21)
# proxmox-backup-client catalog shell host/THUNDER/2026-01-01T11:10:33Z etc.pxar
storing login ticket failed: $XDG_RUNTIME_DIR must be set
Error: Is a directory (os error 21)
# proxmox-backup-client restore host/THUNDER/2025-12-31T16:18:02Z etc.pxar /tmp/test
storing login ticket failed: $XDG_RUNTIME_DIR must be set
Error: Is a directory (os error 21)

I could not find the static binary of the PBS client 4.1, could that be the issue or am I missing something?

 

[Sviat]

Hi,

the problem is that your server is PBS 4.1.0 and your clients are 4.0.16 and 3.4.1.
It looks like starting with PBS 4.1, Proxmox introduced changes to the pxar catalog format, the .didx chunk index handling and manifest structure.
Older clients (especially 3.x and early 4.0.x) cannot correctly interpret these structures.
Just update the clients to 4.1.

 

[Chris]

- https://cloud-pbs.com/wp-content/uploads/2025/06/proxmox-backup-client (3.4.1)

This is not an official source from Proxmox, please be aware of that.

And I get the same error every time, I am able to backup, but I cannot see the catalog or restore. Getting:

storing login ticket failed: $XDG_RUNTIME_DIR must be set
Error: Is a directory (os error 21)

This is how I backup:

# export PBS_REPOSITORY=10.201.3.126:vxnl01a
# export PBS_PASSWORD=XXXX <I have also tried with a file and being prompted for the password>
# proxmox-backup-client version
storing login ticket failed: $XDG_RUNTIME_DIR must be set
client version: 4.0.16
server version: 4.1.0
# proxmox-backup-client backup etc.pxar:/etc
Starting backup: host/THUNDER/2026-01-01T11:10:33Z
Client name: THUNDER
Starting backup protocol: Thu Jan  1 12:10:33 2026
storing login ticket failed: $XDG_RUNTIME_DIR must be set
Downloading previous manifest (Thu Jan  1 12:10:21 2026)
Upload directory '/etc' to '10.201.3.126:vxnl01a' as etc.pxar.didx
Error downloading .didx from previous manifest: Is a directory (os error 21)
[/QUOTE]
[/QUOTE]

This error here indicates that an index file might return an unexpected file type, it being a directory rather than a file.

What filesystem is used to back the datastore on the PBS side? Please check the contents of the directories located at [ICODE]<path-to-your-datastore>/host/THUNDER/[/ICODE], e.g. by [ICODE]ls -lahR .[/ICODE] in that folder.