I found the solution. First you need to get the white pantys. I think on the bottom left side, it will tell you that they are from Julia. Than you have a conversation with Bardie and repeat with the blue ones.
After another conversation you need to clean without panties
If I remember correctly (I'm sure of it ^^ ), the game even tells you what to do exactly.
I tried without pantys first -> game tells me to wear Julias pantys. So I did.
Afterwards the game told me to use Bettie's pantys; I choose the red transparent ones and it worked.
Finally no pantys at all. Seems to be easy.
btw, I also have problems with each update (like the revenge path wasn't startable with v12 for me ... used the save file omeone shared here to make it work) ... but the panty quest seems to be made good and even give hints.
Of course, an exact instruction at the quest log (like other games have) would be extremely helpful for most quests.
Otherwise it's almost impossible to find all new events without a walktrough.
I still hope
DecentMonkey would add such a feature
