From the blackcoin thread :
Downloads
KNOWN BUG: if wallet does not start from first time, ignore error and try again
And the rpc error is triggered if :
-you close solo miner before closing wallet
-you type 'help' or a non existant command in debug console
So it's not that hard to avoid just for today.
If it's triggered you'll have to reboot your computer and this affects only windows and it will be solved before tomorrow night.