- How can I determine what device this is?
You can double-check this device is not present using a list of UUIDs... the commands you've already used to list them are fine.
- Can it be safely removed from startup and if so, how?
You can add a # at the start of the line to comment it out, or create a backup copy of crypttab and then remove the line.
(I like to use etckeeper, it's like having "system restore points" for configuration files in /etc :-) It relies on you knowing Git though).
While researching this I came across several posts that describe similar symptoms but apparently with different causes. Examples of these are this blog entry and this question on Superuser SE, both tracking this issue to a faulty swap partition / entry in fstab. The excerpt from the bootlog however leads me to believe that these are not related to my particular problem.
Fair. It's a very closely related problem though. You have a device which is listed as being required for the boot process. Therefore your boot waits for it. (According to my documentation, the boot process should also require this device, meaning that a failure or timeout will boot into the emergency.target shell, instead of default.target). crypttab even uses options with the same names to control this; noauto, nofail, and x-systemd.device-timeout=....
the boot log, which showed the start job for a device timing out and directly afterwards failing the cryptography setup of a device referenced as cr_usb-General_USB_Flash_Disk. I am unsure whether the two are connected.
They are. (The UUID of the timed-out device is the same as the UUID the crypttab lists as belonging to this USB flash disk).
[ TIME ] Timed out waiting for device dev-disk-by\x2duuid-00a69115\x2d956d\x2d41b3\x2d830c\x2d9a3878087d41.device.
[DEPEND] Dependency failed for Cryptography Setup for cr_usb-General_USB_Flash_Disk_0349315060001623-0:0-part2.
/etc/crypttab
cr_sda2 UUID=7f99168c-4972-468b-900f-fb5bbfb90e66
cr_usb-General_USB_Flash_Disk_0349315060001623-0:0-part2 UUID=00a69115-956d-41b3-83