A group of players I discuss this game with have been experiencing this issue quite a lot.
What we have noticed is that the party always bugs out immediately after a user new to the party joins the party (this happens after quite a number of users are there). That user loads into the party and the house loads and then the screen reloads and that user is sent to the logo screen. After this happens, anyone re-entering the party errors out and the party host gets the invalid party error screen. We've been hypothesizing that this may have to do with the fact that some users will show up at a party but the chat will not list them as ever having shown up (even though those users whose usernames don't appear can access the party normally with all their sims and users there can see that they are the owner of some of the sims at the party). We are suspicious that somehow something is going wrong with the counting behind the number of users who go to the party leading to some users not being counted and the user causing the party to glitch is the user who brought it over the max number of attendees.