I just got this error with Volume 8 now. Seems like every time I binge-play this game it punishes me.
This happened a day after purchasing the speed pass too. Just my luck...
I've already tried everything suggested and nothing has worked. I didn't forsee this would softlock me so I didn't have the foresight to back up my save file.
So here's what happened. I was customizing my Lotus Exige S, every time I tried to apply one of the newer "original" effects it would not apply and the game wouldn't tell me why. I eventually figured out the server was rejecting my changes and I needed to do this before connecting to a lobby. So I did that. And as soon as I did, I started getting errors on every menu telling me to contact customer support and that Item Syncronization failed. I could still edit my car offline but even if I undid all of the changes it wouldn't let me sign back in and I can't click Find Game.
I haven't tried deleting my car yet, I hope I don't have to do so as this is my favorite one.
The error message is, verbatim:
Item System Synchronization failed. Please contact customer support.
FTJR2008
Here is a copy of my savegame if anyone at Criterion wants to have a look: files.catbox.moe/q9z6jn - rename to "1"
Attached to this post is a screenshot of the Driving Effect that caused the game to malfunction.
I refuse to contact customer support as the support website sucks and every time I have used it it has asked me to "resume" my case and then sent me in an infinite loop of creating new ones, it's like they deliberately designed it so you can't get any help.