[SuperSU] JBDegriz:
I have a the same problem with a different file in /system/su.d,

#!/system/bin/sh

logfile0=/data/local/tmp/exfat_mount.log
#logfile=$logfile0
logfile=/dev/null

if [ -e $logfile0 ]; then
logfile=$logfile0
fi

echo '******' $(date) '******' >> $logfile
echo 'id: ' $(id) >> $logfile 2>&1

if [ x$SECONDARY_STORAGE == x ]; then
SECONDARY_STORAGE=/storage/sdcard1
fi

device=/dev/block/mmcblk1p1
echo "Starting for $device" >> $logfile

while read line; do
words=($line)
if [ ${words[0]} == $device ]; then
echo "Error: $device already mounted:" >> $logfile
echo "$line" >> $logfile
exit 1
fi
if [ ${words[1]} == $SECONDARY_STORAGE ]; then
echo "Error: SECONDARY_STORAGE already in use:" >> $logfile
echo "$line" >> $logfile
exit 1
fi
done < /proc/mounts

binpath=/system/xbin

probe=$binpath/probe
if [ ! -e $probe ]; then
echo "Error: $probe not found!" >> $logfile
exit 1
fi

FS=($($probe $device))
echo "$device file system:" ${FS[0]} >> $logfile

# ***** Setup tools for different file systems:
if [ ${FS[0]} == "exFAT" ]; then
echo "$device is exFAT" >> $logfile

mount=$binpath/mount.exfat
if [ -f $mount ]; then
links=( "mkfs.exfat" "fsck.exfat" "dumpexfat" "exfatfsck" "exfatlabel" "mkexfatfs" "mount.exfat-fuse" )
for l in "${links[@]}"
do
if [ ! -e $binpath/$l ]; then
mount -o remount,rw /
ln -s $mount $binpath/$l
echo "Created symlink $binpath/$l" >> $logfile
fi
done
fi

fsck=$binpath/exfatfsck
fsck_ops=

elif [ ${FS[0]} == "NTFS" ]; then
echo "$device is NTFS" >> $logfile
mount=$binpath/ntfs-3g
fsck=$binpath/ntfsfix
fsck_ops='-n'
else
echo "Error: $device is not supported!" >> $logfile
exit 1
fi

# ***** Check if we have required tools.
if [ ! -e $mount ]; then
echo "Error: $mount not found!" >> $logfile
exit 1
fi
if [ ! -e $fsck ]; then
echo "Error: $fsck not found!" >> $logfile
exit 1
fi

# ***** Mount for rw only if the file system is not damaged.
options_ro="ro,uid=1023,gid=1023,umask=0000"
options_rw="rw,uid=1023,gid=1023,umask=0000,noatime"
mnt_cmd="$mount -o $options_rw $device /mnt/media_rw/sdcard1"

$fsck $fsck_ops $device >> $logfile 2>&1
if [ $? != 0 ]; then
mnt_cmd="$mount -o $options_ro $device /mnt/media_rw/sdcard1"
fi
echo "$mnt_cmd" >> $logfile

if [ -e /sys/fs/selinux/enforce ]; then
if [ -e /system/xbin/supolicy ]; then
# ***** Set permissions for fuse. Instead, our mounts would be RO for others.
echo "Using supolicy to provide permissions to fuse." >> $logfile
/system/xbin/supolicy --live "allow sdcardd unlabeled dir { append create execute write relabelfrom link unlink ioctl getattr setattr read rename lock mounton quotaon swapon rmdir audit_access remove_name add_name reparent execmod search open }"
/system/xbin/supolicy --live "allow sdcardd unlabeled file { append create write relabelfrom link unlink ioctl getattr setattr read rename lock mounton quotaon swapon audit_access open }"
/system/xbin/supolicy --live "allow unlabeled unlabeled filesystem associate"
/system/xbin/supolicy --live "allow sdcardd unlabeled filesystem { getattr mount remount unmount }"
/system/xbin/supolicy --live "allow vold unlabeled filesystem { getattr mount remount unmount }"
/system/xbin/supolicy --live "allow init unlabeled filesystem { getattr mount remount unmount }"
else
# ***** Turn SELinux to Permissive. Instead, our mounts would be RO for others.
echo "Using setenforce 0 to provide permissions to fuse." >> $logfile
setenforce 0
fi
fi

# ***** Try to mount:
if [ 0 != 0 ]; then
$mount -o $options_rw $device $SECONDARY_STORAGE
echo "Mounted directly" >> $logfile
else
mount -o remount,rw /system
rm -f /system/bin/debuggerd.mine
echo '#!/system/bin/sh' > /system/bin/debuggerd.mine
echo "$mnt_cmd" >> /system/bin/debuggerd.mine
echo 'start fuse_sdcard1' >> /system/bin/debuggerd.mine
chmod 777 /system/bin/debuggerd.mine
stop debuggerd
mv /system/bin/debuggerd /system/bin/debuggerd.its
mv /system/bin/debuggerd.mine /system/bin/debuggerd
start debuggerd
sleep 2
stop debuggerd
mv /system/bin/debuggerd /system/bin/debuggerd.mine
mv /system/bin/debuggerd.its /system/bin/debuggerd
start debuggerd
mount -o remount,ro /system
vdc volume mount $SECONDARY_STORAGE
echo "Mounted via debuggerd" >> $logfile
fi

The file is called 00mountsd and is used to give access to exfat microsd on stock android 5.1 running on a Chuwi Hi10pro tablet with cherry trail z8350 cpu. I used this on a previous version of the tablet and it worked fine although that tablet came with RemixOS.

The file runs without errors when run manually and succeeds at it task

ls -lZ returns this -rwxr-xr-x root root u: object_r:system_file:s0 00mountsd

I'm running SuperSU Free v2.79 everything else works fine.

Any ideas?