Shutdown problem: mount has 'stop' job queued, but 'start' is included in transaction

In my journalctl, I see the following messages.

• Requested transaction contradicts existing jobs
• mount has 'stop' job queued, but 'start' is included in transaction

There seems to be issues unmounting these two partitions during shutdown:

• /mnt/install (referred to as mnt-install in the log below)
• /mnt/data (referred to as mnt-data in the log below)

How can I fix this, so the partitions are successfully unmounted during shutdown?

I believe these errors prevent my laptop from shutting down. The machine freezes with a black screen, and I have to press and hold the power button to turn it off. I've noticed that when the machine shuts down normally, these messages do not appear in the journalctl log.

Here is an excerpt of the log...

  systemd[1]: Requested transaction contradicts existing jobs: Transaction for colord.service/start is destructive (mnt-install.mount has 'stop' job queued, but 'start' is included in transaction).

  dbus-daemon[637]: [system] Activation via systemd failed for unit 'colord.service': Transaction for colord.service/start is destructive (mnt-install.mount has 'stop' job queued, but 'start' is included in transaction).

  systemd[1]: Requested transaction contradicts existing jobs: Transaction for rtkit-daemon.service/start is destructive (mnt-data.mount has 'stop' job queued, but 'start' is included in transaction).

  dbus-daemon[637]: [system] Activation via systemd failed for unit 'rtkit-daemon.service': Transaction for rtkit-daemon.service/start is destructive (mnt-data.mount has 'stop' job queued, but 'start' is included in transaction).

  systemd[1]: Requested transaction contradicts existing jobs: Transaction for accounts-daemon.service/start is destructive (mnt-data.mount has 'stop' job queued, but 'start' is included in transaction).

  dbus-daemon[637]: [system] Activation via systemd failed for unit 'accounts-daemon.service': Transaction for accounts-daemon.service/start is destructive (mnt-data.mount has 'stop' job queued, but 'start' is included in transaction).

  systemd[1]: Stopped target Host and Network Name Lookups.

  gdm3[775]: Gdm: Failed to contact accountsservice: Error calling StartServiceByName for org.freedesktop.Accounts: Transaction for accounts-daemon.service/start is destructive (mnt-data.mount has 'stop' job queued, but 'start' is included in transaction).