I had this happen last week. I talked to DAS support thinking that was the problem and they essentially stated it was me. I then asked Andrew for his hot-keys just to compare against mine and explained what was happening. Andrew then checked with CMEG and it turns out the problem was on their end.
I should check in here for answers to this stuff more often.
I was trying to stop out a long position on a stock under SSR when this happened, and I had got myself into a bind with 1K shares on something I shouldn't have been doing... It's a horrible feeling. Every time I tried to sell the position, whether it was market order or whatever, it was simply rejected. I restarted DAS, no good. I eventually sold my position by logging in with my iPad and selling the position there (for a gain actually, it moved my way after hitting the stop! yay!).
I still can't short SSR (Tried yesterday) however I have been able to trade stocks under SSR long, but I try to avoid them at least until I know this issue is fixed for good. Andrew has advised me that they are going to fix this but I always have my iPad on the Mobile DAS page ready to log in and get out of the position if needed, and have added CMEG to speed dial!