With some proper logging it could have saved me some gray hair. The cause of all pain was that Java was updated to openjdk11. Old Forge profiles for Minecraft 1.12.2, 1.14.4 needs version 8.
So check your version.
Java -version
openjdk version “11.0.10” 2021-01-19
OpenJDK Runtime Environment (build 11.0.10+9-1)
OpenJDK 64-Bit Server VM (build 11.0.10+9-1, mixed mode)
To fix that one needs to downgrade:
pkg delete -y openjdk11-jre
pkg install -y openjdk8-jre
If your jail is v12 you should also do some mounting from the host. This is for TrueNAS/FreeNAS as host.
mount -t fdescfs null /mnt/red/iocage/jails/mineos-plugin/dev/fd
mount -t procfs null /mnt/red/iocage/jails/mineos-plugin/proc
The mounting needs to be done permanent by modding fstab or something on the host like a postinit script.