My wife has a Sansa e270 and she uses it with MC12 regularly without issue. We did have an issue for a time where somehow the Sansa's filesystem got corrupted, and it wouldn't really crash MC but the sync would never complete. Resetting the Sansa (through the menu) back to it's defaults fixed it. WARNING: This deletes all the data on the Sansa and puts it back to the way it was when you bought it! (But it worked in this case.) I think the corruption was caused by a Firmware update on the Sansa, but I'm not sure.
Other than that -- what type of Playlist are you syncing over? Since it is a Smartlist, not a fixed, manually built Playlist, does it happen if you create a new "manual" playlist with only a few random songs on it? Is it one specific playlist that seems to be doing it, or does it happen if you create a new, identically-filtered smartlist? I'm thinking that maybe: 1) there's something wrong with the list itself, or 2) that there's something wrong (corruption or whatever) with one specific song/file on that list. Have you ensured that each and every file in the list you're trying to sync over actually works/plays?
Another thing that could possibly help track down the issue would be to enable logging, reset the log file, and then make MC crash (following whatever steps cause it). Then look at the logfile created (it's stored at C:\Documents and Settings\<YOUR_USERNAME>\Application Data\J River\Media Center 12\Log.txt), and try to figure out what it was doing just before it crashed.