Mirror problems

[DM]

Since getting ZBrush 1.23b I have been having a problem with mirror in the deformation palette. You will see from this leg that I decided to mirror when I was part way through the modelling process, that the mesh has a kind of tear. This has not happened with previous versions of ZB, if any one has an answer I would be most grateful.
Thanks,
Dave
http://www2.zbrushcentral.com/zbc_uploads/user_image-1010351695uzv.jpg