Wallet maintenance can mean that one, or more several possibilities might be happening.
1) That specific cryptocurrency might have created a fork on its blockchain. To protect your funds, we will disable the wallet until there has been confirmation on which chain is the correct chain.
2) The developer of the wallet has issued an update and COSS.io has to implement the update.
3) The wallet daemon on our server has hung or crashed
4) There's an issue with the wallet that requires the developer to work on it.
Generally speaking, we disable a wallet to prevent deposit or withdrawals from it because it is not functioning as it should be. We advise users to NOT attempt deposits and/or withdrawals of that cryptocurrency when it is under maintenance.