The symptom
You boot from an Ubuntu 25.10 (Questing Quokka) live USB. The installer UI loads — you see the language selection or welcome screen — but then it hangs, shows a connection error, or refuses to proceed.
The screen is up. The backend is not.
The root cause
Ubuntu’s desktop installer uses a two-part architecture:
- Frontend — the graphical UI you see on screen (
ubuntu-desktop-bootstrap) - Backend — the Subiquity server that handles the actual installation logic
They communicate over a Unix socket at /run/subiquity/socket. When the backend is down or unresponsive, the frontend loads but cannot do anything useful. You get a UI with no brain behind it.
The backend runs as a systemd service:
snap.ubuntu-desktop-bootstrap.subiquity-server.serviceIf that service crashes, fails to start, or loses its socket — the installer is dead in the water.
Step 1: Restart the Subiquity backend
Open a terminal (Ctrl+Alt+T or right-click the desktop) and restart the service:
sudo systemctl restart snap.ubuntu-desktop-bootstrap.subiquity-server.serviceCheck if it came back:
sudo systemctl status snap.ubuntu-desktop-bootstrap.subiquity-server.service --no-pager -lYou want to see Active: active (running). If you see failed, inactive, or activating stuck in a loop — continue to Step 2.
Step 2: Check the logs
The logs tell you exactly why the backend is failing:
journalctl -u snap.ubuntu-desktop-bootstrap.subiquity-server.service -b --no-pager | tail -100Common error patterns:
| Log pattern | Likely cause |
|---|---|
socket.error: [Errno 98] Address already in use | Stale socket file — delete /run/subiquity/socket and restart |
ModuleNotFoundError or ImportError | Corrupted snap — reinstall (Step 3) |
PermissionError | Snap confinement issue — reinstall with --classic |
OSError: [Errno 28] No space left on device | Live USB tmpfs is full — reboot or use a larger USB |
| Repeated crash/restart cycle | Corrupted installation media — reflash the USB (Step 4) |
Fix a stale socket
If the socket file is stale:
sudo rm -f /run/subiquity/socket
sudo systemctl restart snap.ubuntu-desktop-bootstrap.subiquity-server.serviceStep 3: Reinstall the snap
If the service keeps failing after restart, the snap itself may be corrupted:
sudo snap remove ubuntu-desktop-bootstrap
sudo snap install ubuntu-desktop-bootstrap --classic
sudo systemctl restart snap.ubuntu-desktop-bootstrap.subiquity-server.serviceThen relaunch the installer:
/snap/bin/ubuntu-desktop-bootstrap --try-or-installIf snap remove says it is not installed, skip straight to the install. If snap itself is broken, restart the snap daemon first:
sudo systemctl restart snapdStep 3.5: Fix broken PAM (deeper issue)
Sometimes the problem is below the installer entirely. If your logs show PAM errors — messages like unable to dlopen pam_*.so, authentication failures, or pam_systemd.so missing — the live session’s PAM stack is broken. Reinstalling the installer snap alone will not fix this.
PAM (Pluggable Authentication Modules) is the authentication layer that everything on the system depends on, including Snap. When a PAM module is missing or misconfigured, it breaks authentication system-wide — not just the installer.
Rebuild the PAM stack:
sudo apt update
sudo apt install --reinstall libpam0g libpam-modules libpam-modules-bin libpam-runtime libpam-systemd
sudo pam-auth-update --forceThen reinstall the snap and relaunch:
sudo systemctl restart snapd
sudo snap remove ubuntu-desktop-bootstrap
sudo snap install ubuntu-desktop-bootstrap --classic
/snap/bin/ubuntu-desktop-bootstrap --try-or-installDiagnose a specific missing module:
If the PAM rebuild does not help, check which modules are referenced but missing:
# List all PAM modules referenced in config
grep -R "pam_.*\.so" /etc/pam.d | sort -u
# List all PAM modules actually installed
ls /lib/x86_64-linux-gnu/security/ | grep pam_If a module appears in /etc/pam.d but is not present under /lib/x86_64-linux-gnu/security/, that is the broken piece. Install the package that provides it — for example, libpam-systemd provides pam_systemd.so.
PAM problems on a live USB are a strong signal that the installation media itself is corrupted, because the live environment should have a working PAM stack out of the box. If the above commands do not repair it, proceed to Step 4.
Step 4: Reflash the USB (nuclear option)
If the service still will not stay up after reinstalling the snap, the most likely cause is corrupted installation media. This is especially common when:
- The ISO download was incomplete or corrupted
- The USB flash tool did not write cleanly
- The USB drive has bad sectors
Download a fresh ISO from ubuntu.com/download and verify the checksum:
sha256sum ubuntu-25.10-desktop-amd64.isoCompare with the official checksum from releases.ubuntu.com.
Reflash with a reliable tool:
# On Linux
sudo dd if=ubuntu-25.10-desktop-amd64.iso of=/dev/sdX bs=4M status=progress oflag=sync
# Or use Balena Etcher on any OSCanonical’s point-release notes frequently mention updated install images that fix installer bugs. A fresh ISO from the latest point release may resolve the issue entirely.
The diagnostic command
If you are stuck and need to share logs for help, this single command captures the most relevant information:
journalctl -u snap.ubuntu-desktop-bootstrap.subiquity-server.service -b --no-pager | tail -50Paste that output in your support request — it contains the exact error that is preventing the backend from starting.
Ubuntu 25.10 context
Ubuntu 25.10 Questing Quokka is an interim release with only 9 months of support. If you are setting up infrastructure for production or long-term use, consider Ubuntu 24.04.3 LTS instead — it is supported until April 2029 (or 2034 with Ubuntu Pro).
The desktop installer (ubuntu-desktop-bootstrap) replaced the older Ubiquity installer starting with Ubuntu 23.04. It is Snap-based and uses the Subiquity backend that was originally built for Ubuntu Server. This architecture is newer and occasionally has edge-case bugs that get fixed in point releases.
Quick reference
# 1. Restart the backend
sudo systemctl restart snap.ubuntu-desktop-bootstrap.subiquity-server.service
# 2. Check status
sudo systemctl status snap.ubuntu-desktop-bootstrap.subiquity-server.service --no-pager -l
# 3. View logs
journalctl -u snap.ubuntu-desktop-bootstrap.subiquity-server.service -b --no-pager | tail -100
# 4. Relaunch installer
/snap/bin/ubuntu-desktop-bootstrap --try-or-install
# 5. Reinstall snap
sudo snap remove ubuntu-desktop-bootstrap
sudo snap install ubuntu-desktop-bootstrap --classic
sudo systemctl restart snap.ubuntu-desktop-bootstrap.subiquity-server.service
/snap/bin/ubuntu-desktop-bootstrap --try-or-install
# 6. Fix broken PAM (if PAM errors in logs)
sudo apt update
sudo apt install --reinstall libpam0g libpam-modules libpam-modules-bin libpam-runtime libpam-systemd
sudo pam-auth-update --force
sudo systemctl restart snapd
sudo snap remove ubuntu-desktop-bootstrap
sudo snap install ubuntu-desktop-bootstrap --classic
/snap/bin/ubuntu-desktop-bootstrap --try-or-installRelated: Linux Distribution Comparison, RHEL 9 for SysAdmins. Need help with Linux infrastructure? Book a consultation.
