Bug in sector 0; -1

Hello. I am really enjoying the game but I found a bug in sector 0; -1 . The mission was to survive 20 waves. I survived 19 waves and now I am stuck at wave 20 because the enemies are stuck in the border of the world in the north. I can't shoot at them but they however can. Of course I restartet the game but the problem was still there.

What can I do? Has anybody the same problem?

(Btw I was playing the 4.0 beta.)