Ubuntu – Tune2fs complaining about “unsupported read-only feature(s)” on fresh ext4 filesystem

I did a fresh install of Ubuntu 16.04. /dev/md1 contains an ext4 filesystem that is mounted at /:

/dev/md1 on / type ext4 (rw,relatime,data=ordered)

Now I want to set the automatic filesystem check interval to one month, but I'm getting a strange error that I find no useful information about:

> tune2fs -i 1m /dev/md1
tune2fs 1.42.13 (17-May-2015)
tune2fs: Filesystem has unsupported read-only feature(s) while trying to open /dev/md1
Couldn't find valid filesystem superblock.

Rebooting didn't help. On another machine running Ubuntu 12.04, the same command works like a charm.

Does anyone have an idea what's wrong?

The command tune2fs -l /dev/md1 works fine, by the way. Here is its output:

tune2fs 1.42.13 (17-May-2015)
Filesystem volume name:   <none>
Last mounted on:          /
Filesystem UUID:          38366a40-bbe1-476b-acfe-d271be6335eb
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index filetype needs_recovery extent 64bit flex_bg sparse_super large_file huge_file dir_nlink extra_isize metadata_csum
Filesystem flags:         signed_directory_hash
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              8380416
Block count:              33521664
Reserved block count:     1676083
Free blocks:              32419430
Free inodes:              8312762
First block:              0
Block size:               4096
Fragment size:            4096
Group descriptor size:    64
Reserved GDT blocks:      1024
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Flex block group size:    16
Filesystem created:       Thu May 31 10:01:27 2018
Last mount time:          Thu May 31 10:49:41 2018
Last write time:          Thu May 31 10:15:14 2018
Mount count:              7
Maximum mount count:      -1
Last checked:             Thu May 31 10:01:27 2018
Check interval:           0 (<none>)
Lifetime writes:          5782 MB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               256
Required extra isize:     32
Desired extra isize:      32
Journal inode:            8
First orphan inode:       4325378
Default directory hash:   half_md4
Directory Hash Seed:      9d55d989-2c2b-4ad8-a29e-53c8798ec68e
Journal backup:           inode blocks
Checksum:                 0x3d9f8d23