maybe you need just to set again the app pool? Configuration → Choose pool
Side note: i’m an urbackup user too, relying on incrementals, and if i may suggest: i have excluded urbackup output dataset at all from snapshot task. Simply reason, they grow really too much for what they offer, and the risk that both those backups get corrupted, and source machine became unvailable (i only take backup of one atm) is for me really far from happen. But is just my personal tradeoff.
This is the resource for pool checkpoints:
https://forums.truenas.com/t/zfs-pool-checkpoints-they-work-just-like-seatbelts-not-really
As suggested in the guide, i take a checkpoint daily (discarding the previous), but be fully aware of the caveouts