CrashPlan as a Jail/Plugin has always been an ever losing battle since its inception. CrashPlan was never meant to be ran on FreeBSD and seemingly every update would break the CrashPlan Jail/Plugin. If you're dead set on running CrashPlan, the best way to do so is on a Linux VM attached to FreeNAS network shares.
Personally, I abandoned CrashPlan months ago for Client-side encryption on Rclone+Google Drive on Google's Gsuite w/Unlimited Storage at $10/mo. No more pain dealing with CrashPlan constantly breaking, my uploads are way faster (300+ Mbps VS <2 Mbps), and Rclone is even built into FreeNAS now (Although I still use a custom jail for my setup).