When walletpassphrase is executed after decryption is removed from memory; iguana gets crashed.
it happens when execute any rpc after walletpassphrase rpc is executed when decyption key is removed.

here is sequence to reproduce that issue;

curl -s --url "http://127.0.0.1:7778" --data "{"agent":"iguana","method":"addcoin","newcoin":"BTCD","services":0,"portp2p":14631}"
curl -s --url "http://127.0.0.1:7778" --data "{"agent":"bitcoinrpc","method":"walletpassphrase","password":"yourpassword","timeout":300}"

at here wait for decryption key to be removed from memory

once the wallet is locked successfully run again walletpassphrase

curl -s --url "http://127.0.0.1:7778" --data "{"agent":"bitcoinrpc","method":"walletpassphrase","password":"yourpassword","timeout":300}"

you should be seeing crash on console.