We have found that VSS Converter crash if the machine on which you are running converter ran out of disk space. VSS Converter requires disk space for storing intermediate data in SQL Express, some disk space for storing the log file and last but not least for downloading the files from VSS before uploading it Team Foundation(Converter use a folder in System drive for it). Hence it is advisable that you ensure that you have 5+GB of spare space on the system drive of machine of which you are running VSS Converter. Here I am assuming you have installed SQLExpress and running VSS Converter from a folder in system drive.
Incase you are using December CTP release you may start migration using incrementatl migration feature otherwise you have to restart migration from scratch.
I also suggest that you ensure that you have good enough diskspace on DT before you start migration.