ReadyNAS 104 system hanging frequently after 6.4.0 upgrade

I'm having the exact same issue.  I also have a ReadyNAS 104.  Mine has 4 x WD Red 3TB disks, and has been working fine until I took the latest upgrade to 6.4.0.

I first noticed the issue when a balance operation (which I had set on a weekly schedule) was still running 24 hours later, and my ReadyNAS was still on, but could not be accessed via ssh or http, and I could no longer access the files on it.  The only option I had was to pull the plug, after which it came up again and tried to do a resync.  I could then access the GUI for about 5 mins, before it hung again and I couldn't access any of the services.  After I while, I figured out that if I cancelled the balance operation by sshing in quickly after booting, before it hung, I could cancel the balance using the btrfs command.  This worked, and after rebooting I was able to get into the GUI without the NAS hanging.  I immediately disabled the scheduled maintenance tasks and deleted a bunch of snapshots and all seemed OK.  But now I still notice that it hangs every few days and I have to pull the plug to reset it, which results in a full volume resync which takes over 24 hours.

It's very frustrating - clearly something is broken with the 6.4.0 firmware (I've tried flashing a new version from a USB stick in case my image was corrupted, but it didn't make any difference), but as per the release notes, you cannot downgrade again from 6.4.0, so I guess I just have to wait for the next update :-(.

Any suggestions gratefully received!