Hi Grep,
Sorry for the inconvenience the issue caused.
We can narrow down the issue is related to the SQL which is not uninstalled correctly. I suspect there are traces from SQL installations left in the registry and file system. If necessary, I suggest you reinstall the SQL server on the computer, and try to install Dirsync with a SQL server to check the issue.
http://technet.microsoft.com/en-us/library/dn441161.aspx
Also, installing Dirsync on a clean install server is an option, or you can install Dirsync on another server that has not installed/uninstalled the SQL. Could you accept the situation?
Thanks,
Johnny Zhang