I was going to ask you about that… The few times we’ve seen this “Unknown internal error” was related to something else bogging the system doing, leading to timeout on some Rockstor operations.
Your mention of docker might be on point here and I wonder if you are in a similar situation as previously reported below:
As a result, you could try having a look at how many subvolumes you have:
btrfs subvolume list /mnt2/<you-pool-name-here>/ | wc -l
The Btrfs usage report is also always a good thing to check, even if it’s unlikely to reveal something wrong in your case:
btrfs fi usage /mnt2/<your-pool-name-here>
Hope this helps,