The failure is repeatable. I use this version:
https://github.com/slimcoin-project/Slimcoin/releases/tag/SLMv0.6.0
version 0.6.0.0-g8e9fe2c-alpha

start the wallet and let it create a new wallet. Send some coins into this new wallet. Then try so forward all or some of this coins, and it fails.