It's not a glitch. Disney is gently and slowly making changes to their system that dis-allow FP+ to be made during a Party Event.
If you DO schedule FP+ on your party ticket, Disney's IT system has been programmed to search out your last day of Park Tickets and cancel all FP+s made on that last day. It might not happen this year, to you, but keep in mind that Disney's new "corporate line" on this matter is that officially, you're not supposed to be making FP+ at the Parties.
The way around this is, cancel all your Party FP+ and try again to schedule your FP+ for your actual last day of PARK TICKETS. If the system is still wacky, you've got to call Guest Services and get them to straighten out/unlock your account.
See, Disney's system isn't smart enough to know which ticket you've scanned in to redeem until you're at the gates. Yes, yes it sees how many tickets you have, but on Party Day,
before you enter the gates, Disney has NO IDEA if you will be redeeming a regular Park Ticket and leaving by 7pm (or whenever the plebes are kicked out

), whether you will be just redeeming your Party Ticket, or even if you will be using your Park Ticket for the day, and switching over to the Party Ticket at 4pm. The FP+ system sees 5 days of tickets, doesn't make a distinction of what type of tickets, but as it knows you have now made 5 days of FP+ (one of those being your Party times), it "knows" there are no more tickets/days/FP+ available to you, so of course it says it can't schedule you more...