I had the same problem when I first got A7. I built a guick small level just to play with the new software and came up with the concave block error, even though I did not see it when I was putting together the level. Never did find a concave block. I deleted the level and so far I have not had a problem with any other builds. It is strange that I did not recieve any warnings until I compiled the level but I figured I did something wrong. I wonder if this has happened to anyone else, maybe A7 is being fickle? Like I said the problem has not come up again. The only way I know how to fix this is to find the concave block and change it.