cannot decrypt pool / all partition tables seem wiped

I am running two pools (8-disk RAID-Z2, a single USB backup disk) with encryption. A few days ago I updated from 11.3u1 to 11.3u2 and motherboard BIOS and IPMI (SM X11SSH-F) as well. I rebooted the system since then several times and decrypted my pools each time sucessfully.

Planning to test/add L2ARC i shut down the system and added a USB3/SSD device and after booting, i cannot decrypt my pools anymore:

I had navigated to Storage, Pools, Add (wrong place to add L2ARC, i know) where i saw my disks all listed as available - see screenshot. I didn't do anything there (Create button was greyed out all time)

I exported the backup pool and tried to import it, then the system said there were no pools to be imported.

I switched to the console and looked at gpart list and inside /dev which only shows partiiton information for the used boot pool. All 8+1 data disks are only listed as the device itself without partition information - if i am not mistaken.

Contact me, you can take a look at the system yourself.

The following information i saved years ago after creating the pools, how it looked like. Exemplary for ada6.

1. check
   zpool status
   pool: vol1
   state: ONLINE
   status: One or more devices is currently being resilvered. The pool will
   continue to function, possibly in a degraded state.
   action: Wait for the resilver to complete.
   scan: resilver in progress since Sat Aug 18 16:44:30 2018
   2.38G scanned at 203M/s, 2.23G issued at 190M/s, 8.88T total
   271M resilvered, 0.02% done, 0 days 13:35:04 to go
   config:

NAME STATE READ WRITE CKSUM
vol1 ONLINE 0 0 0
raidz2-0 ONLINE 0 0 0
gptid/0a1b50e9-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0
gptid/0e9668c5-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0
gptid/132b9fd9-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0
gptid/17d85486-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0
gptid/ab6c8d2b-a189-11e8-a764-ac1f6b641a32.eli ONLINE 0 0 0
gptid/21226aa7-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0
gptid/25b8f273-55f9-11e8-acfb-ac1f6b641a32.eli ONLINE 0 0 0 (resilvering)
gptid/6e254760-5877-11e8-a7db-ac1f6b641a32.eli ONLINE 0 0 0

gpart list ada6
Geom name: ada6
modified: false
state: OK
fwheads: 16
fwsectors: 63
last: 5860533127
first: 40
entries: 152
scheme: GPT
Providers:
1. Name: ada6p1
Mediasize: 2147483648 (2.0G)
Sectorsize: 512
Stripesize: 4096
Stripeoffset: 0
Mode: r1w1e1
rawuuid: 259a8fee-55f9-11e8-acfb-ac1f6b641a32
rawtype: 516e7cb5-6ecf-11d6-8ff8-00022d09712b
label: (null)
length: 2147483648
offset: 65536
type: freebsd-swap
index: 1
end: 4194431
start: 128
2. Name: ada6p2
Mediasize: 2998445408256 (2.7T)
Sectorsize: 512
Stripesize: 4096
Stripeoffset: 0
Mode: r0w0e0
________________________________________________________________
rawuuid: 25b8f273-55f9-11e8-acfb-ac1f6b641a32
________________________________________________________________
rawtype: 516e7cba-6ecf-11d6-8ff8-00022d09712b
label: (null)
length: 2998445408256
offset: 2147549184
type: freebsd-zfs
index: 2
end: 5860533119
start: 4194432
Consumers:
1. Name: ada6
Mediasize: 3000592982016 (2.7T)
Sectorsize: 512
Stripesize: 4096
Stripeoffset: 0
Mode: r1w1e2