I've hosted my weekly party today, and although I should still have 6 h left until its completion, I got a dialog box that told me my party was "no longer valid [Network Error 400]" (see picture).
I usually get my parties up to lvl 10 in the first hour so I can go to bed for class, or start my graveyard shift, and I still get the rewards for it. Every time I open the app, I get the same dialog box popping up and get the party rewards over and over until the time the party should complete - so I've been exploiting that a fair bit lol...
I've also noticed that it tends to happen a few hours into my party, when people on my friends' list are coming on, and like 8 of them join the party at once when there's already a lot of users in attendance :/
While it's fun getting free super and luxe sim tickets and simoleons, it's getting pretty annoying now that 3 of my weekly parties have ended like this. I'm not sure if the other users will get those lvl 10 rewards as well, but I'll ask one of my friends and get back about that.