after upgrade to 18.04 the system sometimes just boots into emergency mode

After I updated my server (no GUI) from 16.04 to 18.04 my system sometimes gets stuck in emergency mode.
Normally my system needs about 20 seconds to get through the dmesg part, and then another 2-5 seconds until the login prompt.

The dmesg part is still fine but, after that, about once a week, I see very slow boot messages dripping down the screen at a rate of 1 line/second.

If this happens, somewhere along the way, the system issues a warning about a device which has timed out like below:

dev-disk-by\x2duuid-db5f4fae\x2d18a0\x2d4df3\x2d8ba3\x2d709f00eac21c.device:
Job dev-disk-by\x2duuid-db5f4fae\x2d18a0\x2d4df3\x2d8ba3\x2d709f00eac21c.device/start timed out.`

and, some time later, there is a message about starting an emergency shell.
But then, the system seems to hang, and I get no prompt nor any console feedback upon typing.

I can issue a Ctrl+Alt+Del though which does a reboot.
After I had done that the system up to now always came up properly in about 20 seconds as stated above and I do not see the device which was complained about before.

I have no clue how I can troubleshoot the issue.
Is this a known problem due to some conflicting conditions in the "glorious" systemd boot mechanisms?
Is there a command like "systemd-analyze blame" which can analyze a failed previous boot?

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: systemd 237-3ubuntu10.4
ProcVersionSignature: Ubuntu 4.15.0-38.41-generic 4.15.18
Uname: Linux 4.15.0-38-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
Date: Fri Nov 9 08:52:32 2018
MachineType: ASUSTeK COMPUTER INC. P9D-X Series
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-38-generic root=UUID=0fa1f208-3a7b-44ca-9f85-e7349a3b1844 ro nosplash
SourcePackage: systemd
UpgradeStatus: Upgraded to bionic on 2018-10-11 (28 days ago)
dmi.bios.date: 10/13/2014
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 0901
dmi.board.asset.tag: To be filled by O.E.M.
dmi.board.name: P9D-X Series
dmi.board.vendor: ASUSTeK COMPUTER INC.
dmi.board.version: Rev 1.xx
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 17
dmi.chassis.vendor: To Be Filled By O.E.M.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr0901:bd10/13/2014:svnASUSTeKCOMPUTERINC.:pnP9D-XSeries:pvrRev1.xx:rvnASUSTeKCOMPUTERINC.:rnP9D-XSeries:rvrRev1.xx:cvnToBeFilledByO.E.M.:ct17:cvrToBeFilledByO.E.M.:
dmi.product.family: To be filled by O.E.M.
dmi.product.name: P9D-X Series
dmi.product.version: Rev 1.xx
dmi.sys.vendor: ASUSTeK COMPUTER INC.