If you used particles in your map go to C:\Feralheart\media\particles and delete any copies.
The copies should have _0 after them. Don't delete any that aren't copies. Sometimes this happens, and I'm not sure why.
I think it can happen in C:\Feralheart\media\sky, too.
If you added new items via C:\Feralheart\media\objects\my_objects, try getting rid of those.
Those are the only other solutions I can think of besides the ones already mentioned.