Chances are that you installed a mod recently that stuffed something up in the inner-workings of the /Blockland folder. Try reinstalling Blockland, but make sure to keep a backup of your saves at Blockland/base/saves.
Best of luck. Sorry i didnt give you a more detailed example, but everytime i open a folder in explorer i get an adware thing wanting to download a virus onto my system, better go deal with that before it does any damage.
Adios.
