When resuming the system from suspend for only a short time, the suspending
again, the system turned off the screen, but failed to power off, but also
failed to respond on keyboard input.
When pressing the hardware power button, the system turned on the screen again
and responded to keyboard input. (Note that the PC power had been on all the
time)

When the system responded again, I inspected the logs. The first related
message seems to be this:
sd-power[2930]: Error setting property 'PowerSaveMode' on interface
org.gnome.Mutter.DisplayConfig: Timeout was reached (g-io-error-quark, 24)
The message repeated after 10 seconds
The these messages:
systemd[1]: Reached target Sleep.
systemd[1]: Starting Suspend...
systemd-sleep[10328]: INFO: Skip running
/usr/lib/systemd/system-sleep/grub2.sleep for suspend
systemd-sleep[10328]: Suspending system...
kernel: PM: suspend entry (deep)
kernel: Filesystems sync: 0.724 seconds
kernel: Freezing user space processes ... (elapsed 0.003 seconds) done.
kernel: OOM killer disabled.
kernel: Freezing remaining freezable tasks ...
kernel: usb 4-3: device descriptor read/64, error -110
kernel: usb usb4-port3: attempt power cycle
kernel: usb 4-3: new full-speed USB device number 13 using ohci-pci
kernel: usb 4-3: device not accepting address 13, error -110
kernel: usb 4-3: new full-speed USB device number 14 using ohci-pci
kernel: Freezing of tasks failed after 20.009 seconds (0 tasks refusing to
freeze, wq_busy=1):
kernel: Showing busy workqueues and worker pools:
kernel: workqueue events_freezable_power_: flags=0x84
kernel:   pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=0/0 refcnt=2
kernel:     delayed: disk_events_workfn
kernel:   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=0/0 refcnt=3
kernel:     delayed: disk_events_workfn, disk_events_workfn
kernel: workqueue writeback: flags=0x4e
kernel:   pwq 16: cpus=0-7 flags=0x4 nice=0 active=0/0 refcnt=4
kernel:     delayed: wb_workfn, wb_workfn
kernel: workqueue usb_hub_wq: flags=0x4
kernel:   pwq 0: cpus=0 node=0 flags=0x0 nice=0 active=2/0 refcnt=3
kernel:     in-flight: 6573:hub_event [usbcore] hub_event [usbcore]
kernel: workqueue xfs-reclaim/dm-8: flags=0xc
kernel:   pwq 6: cpus=3 node=0 flags=0x0 nice=0 active=0/0 refcnt=2
kernel:     delayed: xfs_reclaim_worker [xfs]
kernel: pool 0: cpus=0 node=0 flags=0x0 nice=0 hung=0s workers=5 idle: 5597
5501 6705 6539
## While the following messages still have the date and time of suspend, I
guess that refer to the resume already
kernel: Restarting kernel threads ... done.
kernel: OOM killer enabled.
rtkit-daemon[2812]: The canary thread is apparently starving. Taking action.
/usr/lib/gdm/gdm-x-session[3048]: (II) event4  - Dell Dell USB Entry Keyboard:
SYN_DROPPED event - some input events have been lost.
kernel: Restarting tasks ... done.
kernel: PM: suspend exit
kernel: PM: suspend entry (s2idle)
rtkit-daemon[2812]: Demoting known real-time threads.
rtkit-daemon[2812]: Demoted 0 threads.
## the following is the first message with a time stamp matching power on
kernel: Filesystems sync: 0.235 seconds
kernel: Freezing user space processes ... (elapsed 0.003 seconds) done.
kernel: OOM killer disabled.
kernel: Freezing remaining freezable tasks ...
kernel: usb 4-3: device not accepting address 14, error -110
kernel: usb usb4-port3: unable to enumerate USB device
kernel: (elapsed 9.671 seconds) done.
kernel: printk: Suspending console(s) (use no_console_suspend to debug)
kernel: parport_pc 00:05: disabled
kernel: serial 00:04: disabled
kernel: r8169 0000:02:00.0 eth0: Link is Down
kernel: sd 0:0:0:0: [sda] Synchronizing SCSI cache
kernel: sd 1:0:0:0: [sdb] Synchronizing SCSI cache
kernel: sd 0:0:0:0: [sda] Stopping disk
kernel: sd 1:0:0:0: [sdb] Stopping disk
kernel: serial 00:04: activated
kernel: parport_pc 00:05: activated
kernel: sd 1:0:0:0: [sdb] Starting disk
kernel: sd 0:0:0:0: [sda] Starting disk
kernel: r8169 0000:02:00.0 eth0: Link is Down
...
kernel: r8169 0000:02:00.0 eth0: Link is Up - 1Gbps/Full - flow control off
...
kernel: OOM killer enabled.
kernel: Restarting tasks ... done.
kernel: PM: suspend exit
kernel: usb 6-3: USB disconnect, device number 3
## ... all the messages above have the same timestamp
## the message following have a time stamp one second _before_ the messages
above
rtkit-daemon[2812]: The canary thread is apparently starving. Taking action.
systemd-sleep[10328]: System resumed.
rtkit-daemon[2812]: Demoting known real-time threads.
gsd-power[2930]: Error setting property 'PowerSaveMode' on interface
org.gnome.Mutter.DisplayConfig: Timeout was reached (g-io-error-quark, 24)
rtkit-daemon[2812]: Demoted 0 threads.
systemd[1]: Starting Rotate log files...

So could an USB problem cause that?

I found some USB problem for booting, but the devices are the same for a long
time; maybe a kernel bug?

systemd[1]: Started Tell Plymouth To Write Out Runtime Data.
kernel: usb 4-3: device descriptor read/64, error -110
systemd-cryptsetup[700]: Set cipher aes, mode cbc-essiv:sha256, key size 256
bits for device
/dev/disk/by-id/ata-Samsung_SSD_860_EVO_500GB_S4XBNF0M929229V-part2.
...
systemd[1]: Mounted /home.
kernel: usb 4-3: device descriptor read/64, error -110
kernel: usb 4-3: new full-speed USB device number 4 using ohci-pci
kernel: usb 4-3: device descriptor read/64, error -110
systemd-udevd[582]: seq 2193 '/devices/pci0000:00/0000:00:12.1/usb4' is taking
a long time
systemd-udevd[582]: seq 2216 '/devices/pci0000:00/0000:00:13.1/usb6/6-2' is
taking a long time
kernel: usb 4-3: device descriptor read/64, error -110
kernel: usb usb4-port3: attempt power cycle

The interesting thing is that I do not have a device at usb 4-3 it seems:
SysFS BusID: 4-2:1.0 and SysFS BusID: 4-2:1.3 is an USB headphone with
microphone ("C-Media Electronics Audio Adapter (Unitek Y-247A)")

# lsusb
Bus 007 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 002: ID 05e3:070e Genesys Logic, Inc. USB 2.0 Card Reader
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 006 Device 004: ID 413c:2107 Dell Computer Corp. 
Bus 006 Device 002: ID 046d:c00e Logitech, Inc. M-BJ58/M-BJ69 Optical Wheel
Mouse
Bus 006 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 004 Device 002: ID 0d8c:0014 C-Media Electronics, Inc. 
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub