Solved, see below.
I recently reinstalled my home server and was unable to open my LUKS-encrypted hard drive. Neither my usual passphrase nor a newly created keyfile were working. I tested on different distros, initially on my new Proxmox installation, later on a the Arch ISO. I eventually tried the disk on my main system, on which it used to be and I still had an old keyfile on - et voilá. So I created keyfiles as suggested in the wiki and occasionally md5sum returned a different hash for the keyfile! Why is this happening? I find' this extremely concerning because this could potentially result in massive data loss due to a keyfile apparently randomly not working as I was experiencing it. What am I missing?
For reference because I don't know how to share what I exactly did.
Scenario #1:
A directory on a mounted hard drive on my desktop.
$ echo -n '$mypassphrase' > ./dir/keyfile
$ md5sum ./dir/keyfile
c6dd9329dbe030127ce5e19d85de4df9 ./dir/keyfile
# chown root:root ./dir/keyfile; chmod 400 ./dir/keyfile
# md5sum ./dir/keyfile
c6dd9329dbe030127ce5e19d85de4df9 ./dir/keyfile
Scenario #2:
My old keyfile in /etc on my desktop containing $mypassphrase.
# md5sum /etc/keyfile
a1c10c2d023c982259f6c945ebee664e /etc/keyfile
Scenario #3:
Booted from the Arch ISO on my server.
# echo -n '$mypassphrase' > keyfile
# chown root:root keyfile; chmod 400 keyfile
# md5sum keyfile
c6dd9329dbe030127ce5e19d85de4df9 keyfile
Scenario #4:
A directory in /home on my desktop.
$ echo -n '$mypassphrase' > ./keyfile
$ md5sum keyfile
a1c10c2d023c982259f6c945ebee664e keyfile
EDIT: I just moved the disk back into my server and tried echo'ing my passphrase into a keyfile which returned the hash starting with c6, whereas opening a file using nano and pasting the passphrase into the file returned the hash starting with a1.
EDIT: I moved the disk back into my server, reinstalled Proxmox and tried again. I was able to unlock the disk after I pasted the passphrase into a file and deleted all trailing spaces/newline. I also tried echo'ing the passphrase into a keyfile and that also did not work, no clue why but it seems to work on some systems on not on others.
Try od (or hd) to compare short files precisely visually.