Ubuntu – New hardware – system does not boot anymore – how to reset systemd/udev

I have Ubuntu 16.04. I just replaced the hardware (new motherboard, new CPU, new GPU, new RAM, new network card, new SATA controller) and now Ubuntu doesn't boot up anymore. After a while, I get into the emergency shell and it seems I have full access to the system there. I was able to manually setup the network (ifconfig en5ps0 up; dhclient en5ps0 <- very strange interface name it came up with). When looking at the journal (journalctl -xb), it seems like it got a timeout while waiting for some disk it does not find anymore.

Some maybe relevant journal messages:

...
systemd: Received SIGRTMIN+21 from PID 2816 (plymouthd).
...
root: /etc/dhcp/dhclient-enter-hooks.d/avahi-autoipd returned non-zero exit status 1
...
root: /etc/dhcp/dhclient-enter-hooks.d/samba returned non-zero exit status 1
...
systemd: Received SIGRTMIN+20 from PID 3100 (plymouthd).
...
systemd: dev-disk-by\x2duuid-....device: Job dev-disk-by\x2duuid-...device/start timed out.
systemd: Timed out waiting for device dev-disk-by\x2duuid-....device.
-- Subject: Unit dev-disk-by...device has failed
...
-- The result is timeout.
systemd: Dependency failed for File System Check on /dev/disk/by-uuid/....
...
systemd: Dependency failed for /mnt/....
...
systemd: Dependency failed for Local File Systems.
...

/etc/fstab:

root@gcomputer:~# cat /etc/fstab 
# /etc/fstab: static file system information.
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
proc            /proc           proc    defaults        0       0

#/dev/sdb1: UUID="56fc92d2-1903-4263-b88e-d09bc15ef1d3" TYPE="ext4"
#/dev/sdb2: UUID="f9b799de-c564-4e00-9924-4e8a0ffe8d51" TYPE="swap"

# new SSD (OCZ-VERTEX2_OCZ-K5Q40019666QDZLM)
UUID=56fc92d2-1903-4263-b88e-d09bc15ef1d3 /               ext4    discard,noatime,user_xattr,acl,relatime,errors=remount-ro 0       1

# /dev/sdb5:
# UUID=78ca7a27-6fcc-493c-a10a-5ed961a682e5 none            swap    discard,sw              0       0


/dev/scd0       /media/cdrom0   udf,iso9660 user,noauto,exec,utf8 0       0

# very old Gentoo (ST380011A_5JVHHAX0)
UUID=64f30a2f-1c38-40e8-8ab2-7f639b9c3673   /mnt/gentooroot reiserfs    user_xattr,acl  0 1
UUID=a448006c-43df-4fbe-be3d-18da22b4e29c   /mnt/gentooroot/home    reiserfs    user_xattr,acl  0 1

# oldroot (WDC_WD5000AACS-00G8B1_WD-WCAUK0065639)
UUID=2474adbe-ca12-4ad1-bea1-1938fdb1c8a4   /mnt/oldroot            ext3    noatime,user_xattr,acl,relatime,errors=remount-ro 0       1

blkid:

root@gcomputer:~# blkid
/dev/sda1: UUID="56fc92d2-1903-4263-b88e-d09bc15ef1d3" TYPE="ext4" PTTYPE="dos" PARTUUID="2db0af09-01"
/dev/sda2: UUID="f9b799de-c564-4e00-9924-4e8a0ffe8d51" TYPE="swap" PARTUUID="2db0af09-02"
/dev/sdb1: UUID="2474adbe-ca12-4ad1-bea1-1938fdb1c8a4" SEC_TYPE="ext2" TYPE="ext3" PARTUUID="00039d20-01"
/dev/sdb5: UUID="78ca7a27-6fcc-493c-a10a-5ed961a682e5" TYPE="swap" PARTUUID="00039d20-05"

So, how do I fix this? Why does it even wait for the disk? The root filesystem including home and everything which it needs is already available.

Do I need to tell udev or systemd or whatever to rescan for new hardware or for new disk ids or so? Why doesn't it do that automatically?

How would I debug that further?