Found the error. This command pct set 114 -mp0 mp=/mnt/sdb,/root/mnt Takes as the first mp /mnt/sdb the lxc file placement, and the second /mnt/sdb the host folder. These where switched.
The original posted, which is wrong: pct set 114 -mp0 mp=/mnt/sdb,/root/mnt
The new, which works: pct set...