[Nexus 5 Original Android Development] Tasssadar:
http://review.cyanogenmod.org/#/c/56469/

restorecon -R /data runs and changes SELinux contexts in whole of /data -> /data on secondary ROMs contains /data/media/0/multirom/roms/, which in turn contains all data of said ROM, including /system and /cache -> restorecon changes contexts of files in /system as if they were in /data, rendering them unaccessible -> crashes. Example output of restorecon -vR /data, to demonstrate what I mean:

root@hammerhead:/ # restorecon -Rv /data
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/system/bin/.ext/.su from u:object_r:system_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/system/etc/install-recovery.sh from u:object_r:system_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/system/xbin/su from u:object_r:system_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/system/xbin/daemonsu from u:object_r:system_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/cache from u:object_r:cache_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/cache/recovery from u:object_r:cache_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/cache/lost+found from u:object_r:cache_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/cache/dalvik-cache from u:object_r:dalvikcache_data_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/media/0/multirom/roms/BeanStalk-4.4.205-20131232/cache/backup from u:object_r:cache_backup_file:s0 to u:object_r:system_data_file:s0.
Relabeling /data/data/com.android.backupconfirm from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c0,c256.
Relabeling /data/data/com.android.backupconfirm/lib from u:object_r:system_data_file:s0 to u:object_r:app_data_file:s0.
Relabeling /data/data/com.android.providers.calendar from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c1,c256.
Relabeling /data/data/com.android.providers.calendar/lib from u:object_r:system_data_file:s0 to u:object_r:app_data_file:s0.
Relabeling /data/data/com.android.providers.calendar/cache from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c1,c256.
Relabeling /data/data/com.android.contacts from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c2,c256.
Relabeling /data/data/com.android.contacts/lib from u:object_r:system_data_file:s0 to u:object_r:app_data_file:s0.
Relabeling /data/data/com.android.providers.contacts from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c2,c256.
Relabeling /data/data/com.android.providers.contacts/lib from u:object_r:system_data_file:s0 to u:object_r:app_data_file:s0.
Relabeling /data/data/com.android.providers.contacts/cache from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c2,c256.
Relabeling /data/data/com.android.defcontainer from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c3,c256.
Relabeling /data/data/com.android.defcontainer/lib from u:object_r:system_data_file:s0 to u:object_r:app_data_file:s0.
Relabeling /data/data/com.android.dialer from u:object_r:platform_app_data_file:s0 to u:object_r:app_data_file:s0:c4,c256.

Problem is, I have absolutely no idea what to do about that. That script runs as root, so I can't prevent it from changing contexts in /data/media/0/multirom by setting scrict permissions to that directory. I'm gonna have to disable that script, most likely. Give me a bit more time, I'll try to come up with something better, but I really have no idea.

PS: It's okay after reboot, so at least there's that.