So in the second sex scene in the dungeon under the ruins of a temple I got one CG and than a black screen is it a bug or is there just no CG yet?
The CG is already in the game. Did you have a black screen but the text kept playing until the event was over? Maybe you used the update files and missed one update? You could check in the www/img/pictures folder if you have the images Mira-Second-Sex-PNG-01 from number 01 to 08.
If you're missing files the safest thing to do is to re-download the full game again and move your save files from the old game folder to the new one.
I have a video about how to use update files. where I also explain how to move save files at about 1:27: