@1g333 Did you also delete the localthumbcache file? If not you could try deleting that, then run a game repair, then open the game in a brand new save and try with another sim in that new + vanilla save. If you can trigger the event and get the task checked off in that game it should also be completed once you go back and open your other save.
Also, did you click the event task tab area and see if the task got checked off when one of those ghosts appeared, but you didn't get/see a notice about that? My event task for the ghost/bar checked off during one of my attempts where I did not see a ghost yet. After that I re-tried that night (went back to a save I made before leaving for the bar), and I did see a ghost the next time, but I didn't get any notice that it checked off since it had already done that before. I was more than a little confused.