The earlier posts in dw_maintenance have some information on that! Because LJ is under DDoS and working to mitigate the attack, and because the site isn't always reachable, crossposting is going to fail more often than not. Even if you can reach LJ from your computer, that doesn't mean that DW can reach LJ at that given moment. The best thing to do is to just leave it be once the crosspost fails, and retry in a few days (by editing the post and checking the crosspost box) once the overload on LJ eases up a bit.