As Ezio said, it may be that a character is inside your map, however, I can say that it hasnt made a difference if a charrie is in a map or not at the time of exporting.
If I get this problem, all i do is close the FH window, then try again, most of the time this works.
If this dosn't work, then you may be missing a vital piece of your map such as a .map file or such, it's best to check if you have accidently deleted any of these.
And a final reason I can think of is that there is either a mesh or particle that is not exporting properly due to missing files of its own, causing the map to fail exporting, so try fishing through your files to see if you can find which one may be causing the problem and either restore the missing file/s or delete the object from your object group.
I hope you manage to sort this ^^