I think you need 2 colons after the group name in the group file. Here is what each line should contain:
<group name> : <group password> : <gid> : <user>,<user>,<user>
if you have something between the first and second delimiters then that becomes the encrypted password for the group.
I think your system is expecting a password tied to the groups and when it gets no password it fails.
try looking at the man pages:
man 5 group
man 5 passwd
They explain the format very well