well, I was still in a testing phase so no worries, but I installed a new boot device and a fresh version of freenas. (old boot device was 11.2 and the new 11.3)
the new boot, didn't recognise the old pool; then I re-booted the previous version and properly exported the pool. Afterwards it was possible to import it into the new installation.
But some folders didn't show up on my client system, even though they were still there. Some folders also has (ACL) behind them.
Anyway, I was still testing so I just deleted everything and started a fresh pool.