I had a similar experience about a month ago. Again the Dax. I was up over Euro300. Tried to close it out repeatedly. Despite many efforts I watched that nice little profit turn into a loss of over Euro140 when I was finally stopped out.
I rang them and they said they'd check into it.
After many many emails where I was continually fobbed off, I gave up. I never got a reply from the same person twice.
In a fast moving market, which the DAX can be often, I think their platform is unstable - but that, of course, they will not admit. A couple of times they suggested it was a browser problem, i.e., a problem at my end. "Blame the victim".
If you try to get on to a fast moving market on IG - their platform won't let you. I realise there is a latency issue there. But it also seems to me that in a fast moving market and you have already entered before the fast move started it is often difficult to get off. Of course, while it is screaming up - you don't want to get off, it's only when it turns around and starts to come down is it prudent to pull out. But then - their platform doesn't seem to allow you to get off.
In this particular instance, I received several messages, e.g., Close Position Error, and, most commonly, This market is being rolled over and is currently unavailable.
They told me that those messages only occur after a position has been closed. But I kept getting them as I frantically tried to close the position, and watched while a saw a nice little profit turn into a loss.
(20min delay)