I had same issue last time I reinstall D2 LoD. After this message I restarted my PC and re-tried connecting to battlenet. On second attempt, it managed to update Bnet successfully and this issue was gone. Give it a couple tries, for some reason sometimes Bnet will not autoupdate to newest version and you would see this message. I hope it would work for you as well.
Regards,
Rolitasr.