For me, the issue was a total break of SSH. I’ve heard it’s broken Time Capsule backups as well.
So, please see: https://download.wdc.com/nas/My_Cloud_BWVZ_2.31.183.bin
I have a Mirror Gen2. As the only available firmware on the site is the latest, I used the existing URL and changed ‘.193’ to ‘.183’. Downloaded fine and was able to be installed using the manual upgrade button in the GUI.
Hope this helps anyone else to find themselves in this position.
Oh yeah, turn Firmware Auto Update OFF. Not worth being the canary in the calming when this sort of thing happens.