Finally, I managed to fix it!
(I swear, I've tried everithing, before.)
That's how I did:
1) saved a view, from 3d window;
2) in saved view, in the view settings, I was able to set the renovation status (originally it showed a "missing ren. status" (the 3d window has been stucked in a missing status); and at first I was unable to change it; but at second try, it worked);
3) all these procedure freed the inactive status in 3d window, so it works again.
Solved!
(I think it can help others, that's why I'm not deleting)
Talmácsi, István, architect (AC user since 1997, ac4.5 - now: ac18)