In the meantime I found a solution on my own. One has to make the process wait for the iSCSI device to appear. By default this makes
/etc/rc.local inappropriate but wait, there's process forking!
This is my complete
/etc/rc.local and it works nice. When the graphical login manager appears it still takes 1-2 seconds for the filesystem to be mounted, so don't type in your password too fast.
# This script is executed at the end of each multiuser runlevel.
# Make sure that the script will "exit 0" on success or any other
# value on error.
# In order to enable or disable this script just change the execution
# By default this script does nothing.
# wait for mount device to appear
until [ -e /dev/mapper/iscsi_crypt ]
# try to mount once and exit