"Sorry, this transaction is no longer in your memory pool"-what does it mean? The balance remained zero. A mining reward that was not confirmed by the rest of the network?
Those are the transactions in the memory pool, not coins generated. Generated coins will appears in the transactions tab. There is a possibility that you mined an orphan block and it got rejected by the network. Basically someone found a block just before you and the network validated the other block.
Yes, the message I gave is shown when trying to find out the details of a transaction that appeared in the transaction tab in the wallet.