The UNUSED_SPACE message signifies a pointer to a memory region that is not supposed to be in use.  Yet, you obtained its class successfully as String which suggests that the memory was at some point mmapped.  Try running with verbose level 2 (I think) or higher to get JMTk to dump the address ranges it uses for various spaces to check if the bad pointer is a live(?) object still in from-space after a collection.  In addition, consider enabling the flag PROTECT_ON_RELEASE to munmap the supposed dead regions (from-space after collection before flip) of memory.