Oh, and copy & paste between two opened maps does not work. And opening a map when DCME is open (by double-clicking) causes DCME to give an index OOB error.
Most likely it is a problem with your video card drivers. Update them. If that does not help, try and see if control panel->administrative tools->event log->system says something about the shutdown.
Well, first it happened when I just edited a map and saved it (no details). But now when I try to import a tileset to this broken map, it also fails! Just open map, import tileset and save.
One more: sometimes it saves a completely black tileset! (I think it's an invalid BMP, actually. Anyway, it shows up totally black in DCME and Continuum). Like this: xduel.zip
Do you have some downloads going on while playing? They can cause packetloss. Otherwise, just wait it out. It might take a few weeks or more but it should fix itself if it is just a random internet problem. You can also do a trace to a zone (let it go at least 100-200 cycles) and paste the results here - maybe that will give some info about the cause.