https://medium.com/@AugurProject/serpent-compiler-vulnerability-rep-solidity-migration-5d91e4ae90ddThe REP contract and the contracts in development where written in the programming language Serpent. They used this language because they started before solidity became popular. As part of their development process they had the Serpent compiler audited and in the process uncovered bugs in the Serpent compiler that critically affected their ERC20 token in a way that would prevent transfers.
So in response they deployed a new contract for the REP token written in solidity and intentionally used the exploit they found to halt the original REP contract. If you have REP at an address you will have it at the same address in the new contract. You may have to tell your client to watch this new address for REP tokens or rescan for balances in some clients.
Hopefully when they Audit the Solidtity compiler they dont find bugs in it as well.