Thanks Rotund! I am doing it exactly like you recommended now - like I just wrote in another thread:
Well - for now I disabled autosave - and it has some real advantages:
Auto-save does not block my DAW in a critical moment (after hitting stop to hear something again)
Instead of that I am NOT interrupted in my flow anymore
I can hit ctrl-s while I am actually listening to something, because playback is not interrupted during those 50 sec
Regarding the bac file… well, I tried of course everything. Renaming etc… Cubase… Nuendo on the Mac… I even sent that file to E.Doll - it seems that these files were “zeroed out” - size etc is fitting (about 85MB in this case) - I checked in a Hexeditor - while a working file has a lot of bullsh*t in it that bac file had only Zeros … though I have no clue about Hexeditor stuff it seems to me that those files are NOT identically/similar 
Learned my lesson.