I get a crash as well on 4.2.1 but only if I sync the Game folder. All subfolders sync fine.
The error in the output log I get is:
[2014.06.30-15.04.24:644][370]LogWindows: === Critical error: ===
Unhandled Exception: 0x00000001
[2014.06.30-15.04.24:644][370]LogWindows: LongPackageNameToFilename failed to convert ‘/Game’. Path does not map to any roots.
Address = 0xfcea940d (filename not found) [in C:\Windows\system32\KERNELBASE.dll]