what does receivership mean in this context?
It means, most likely, that they will submit themselves to an independent third party auditor who will act in the position of arbitor/judge in the effort of distributing the assests that remain as equitable as is possible.
And I wonder how large was "a large amount"
There were four wallet pools that operated mostly automaticly, so it's roughly one quarter of the total deposits, I would guess. If there were more pools, independently secured, then the loss ratio would be lower; and if there were only one pool the loss would be total. This is a good lesson for future online wallet websites to learn from. Many wallet pools with independent security is better than less. Best would be something like a single wallet file per user, with a method of single user encryption. Perhaps something that requires the user to log into his account, and then provide a different
passcode to decrypt the user wallet onto a ramdisk for a limited period of time. Alternatively, an android client could use public/private keys like SSH does to double authenticate the user account, thus requiring that an attacker have access to the individual user's phone, home computer or other device with that secret key, and the phone client can reprovide the server a decryption key every 15 minutes or so. In this way, the user can conviently use the wallet service on his phone when Internet is available to him, but whenever he closes the client, turns off his phone or leaves the range of his cell service his wallet.dat file is only exposed for (at most) the next 15 minutes until the file is purged from the ramdisk. A cracker breaking in could (potentially copy the wallet.dat files from the ramdisk, as well as copy as many of the encrypted wallet.dat files as he can find before being noticed, but the encrypted wallet.dat files are useless because his intrusion is unlikely to go unnoticed for long (as this event illustrates) and users can be notified that they need to flush their accounts and then destroy their wallet.dat and not reuse those keypairs again. These steps would limit the exposure to risk of the membership of an online wallet service, regardless of the degree of security.