3 months later I realize that this could not have solved my problem, because I added it to /boot/refind_linux.conf where it does not belong at all and actually results in an error message during boot 
x-systemd.device-timeout=90 is a systemd-specific mount option that has to go into /etc/fstab to work, if I understand this and that correctly.
I guess the problem went away by itself so I’ll remove the solution flag, sorry 