Hi, dhchentw!
I did a bit more digging and it seems that (at least in my case) the problem arises because of non-ascii symbols in OU names, not group names. Could you please check what OUs the users are part of?
I also did a bit more searching for LW_ERROR_STRING_CONV_FAILED and judging by some Ubuntu related posts, it seems to be an error originating from likewise-open. It seems this is somehow connected to the system locale:
https://bugs.launchpad.net/ubuntu/+source/likewise-open/+bug/719279
https://bugs.launchpad.net/ubuntu/+source/likewise-open/+bug/598034
https://bugs.launchpad.net/ubuntu/+source/likewise-open/+bug/794108/+activity
However, none of the workarounds proposed in those links are working for me.