In the same location as the mdb (master database) file, is there another file with an ldb (lock database) extension? This file is created when modifications are being made to the toolbox and then deleted when finished, however I have seen an ldb file not get deleted and prevent the upgrade from moving forward properly. Deleting that file may be the solution, but without further research, I can't recommend deleting it without first being sure that there is nothing in your toolbox that actually is/was modified, as you may lose any of those changes if the ldb file is deleted.
I would start there, and if you see an ldb file then that is probably what's holding you back, but I would definitely double check before deleting it.
Derek M. Lawson