The kernel is standard. No customizations. Pertinent parts of uname -a : Linux 3.10.0-1160.25.1.el7.x86_64 #1 SMP x86_64 GNU/Linux
Running with -d showed a lot of lines with nothing problematic except the last line:
FATAL Master() container creation failed: mount /proc/self/fd/3 -> /usr/local/var/singularity/mnt/session/rootfs error: while mounting image /proc/self/fd/3: squashfs filesystem seems not enabled and/or supported by your kernel
/proc/self/fd/3 -> /var/lib/sss/mc/passwd
ls -al /var/lib/sss/mc/passwd : -rw-rw-r-- sssd:sssd 8825056
/usr/local/var/singularity/mnt/session existed. There was no subdirectory rootfs. So I created one. Still fails.
dmesg has no reference to squashfs.