Lets say you want to encrypt more than one disk at once. You choose ada1, algorithm, length and the password, tick 'Initialize and encrypt disk and then 'add'. Then it works, geom data written etc bla. Everything alright so far. You have your ada1.eli device.
Now you choose the next disk 'ada2', choose the same things again and when you click on add, it still uses ada1 and this makes the bug, so n4f wants to encryt the already encrypted device. -> error and you can recreate it all over again.
The workaround, when you encrypt ada1 and geom data is written, you have to go to 'management' and then add the second disk.
It's a bit irritating and frustrating at the first time and I think it's newbie trap as it happened to me.