Hello sslegi,

backup you're wallet.dat just to be sure and close the wallet.
In "%APPDATA%\Whitecoin" delete everything except "wallet.dat" and download the latest blockchain snapshot here: https://github.com/Whitecoin-org/xwc-blockchain/releases/download/latest/xwc-blockchain.zip
unpack this inside "%APPDATA%\Whitecoin" and start the wallet again.