got this same problem, really wanted to use this but it's completely broken because of this issue
So to make a lot of troubleshooting short, its the animation for the breasts scale/positioning. due to unity's system, it is looking for the dynamic bones version of the animation and glitches when it finds the physbones version vcc updates to. I just deleted that animation file from the project and it went back to normal (except for not having that toggle now)
Ahhhhhhh gaddangit VRC why'd you have to go and change jigglesystems in a way that always breaks all the old ones! Grrrr.
Thanks a bunch for the troubleshooting and for letting me know exactly what was going on :) Greatly appreciate it, and I really gotta find a free week once I finish commissions at long last to just sit down and upgrade allll the existing models @_@ so many of them.