It's bug 159 in the new bug tracker and in your scenario I would do these 2 steps from the workaround:
rm /etc/gshadow
grpconv
Before doing it you might want to open /etc/gshadow with an editor to see whether you have any lines looking badly like this:
shared:x:

,w,,,,,,,,,,,,,x,d,w,,x,x,x,w,x,w,,,,,,,,,,,,,x,d,w,,x,x,x,w,x,w,,,,,,,x,x,x,w,x,w,,ww,dmin,,admin,n,,ww,dmin,,ww,dmin,x,d,w,,x,x,x,w,www,admin,public
Michael