According to the man page cgcreate is supposed to use the owner of the parent cgroup if no user is specified with the -a or -t options. This worked as intended in libcgroup 0.37 because the uid/gid variables where initialized to CGRULE_INVALID (-1), which is a special value for the chown() syscall to leave the ownership unchanged.
Commit 3ccbaef introduced a new special value NO_UID_GID, which is also -1 and maps to owner/group 0 in cg_chown. So, starting with libcgroup 0.38 the cgcreate command fails when a non-root user tries to create a group without specifying the -a and -t options. The new group is successfully created, but the exit code of cgcreate is 96:
cgcreate -g cpuacct:/test/mine
cgcreate: can't create cgroup /test/mine: Operation not permitted