Accidentially I noticed that one of my two MD-RAID1 is flagged "auto-read-only"
for no obvious reason. Also no insightful message in journal.
I have two IMSM RAID1: 2x2TB disks (sda,b), 2x250GB disks (sdc,d)
The MD_STATUS is like this:
~> cat /proc/mdstat 
Personalities : [raid1] 
md124 : active (auto-read-only) raid1 sdc[1] sdd[0]
      244195328 blocks super external:/md125/0 [2/2] [UU]

md125 : inactive sdc[1](S) sdd[0](S)
      6248 blocks super external:imsm

md126 : active raid1 sdb[1] sda[0]
      1953497088 blocks super external:/md127/0 [2/2] [UU]

md127 : inactive sda[1](S) sdb[0](S)
      6306 blocks super external:imsm

unused devices: <none>

Messages reladed to md124 in journal are essentially these since boot:
Jan 27 19:19:50 host kernel: md/raid1:md124: active with 2 out of 2 mirrors
Jan 27 19:19:50 host kernel: md124: detected capacity change from 0 to
250056015872
Jan 27 19:19:50 host kernel:  md124: p1

Even in the previous boots I found nothing unusual (the RAID is without problem
in Windows).

mdadm shows this status:
# mdadm -Q --detail /dev/md124
/dev/md124:
         Container : /dev/md/imsm1, member 0
        Raid Level : raid1
        Array Size : 244195328 (232.88 GiB 250.06 GB)
     Used Dev Size : 244195460 (232.88 GiB 250.06 GB)
      Raid Devices : 2
     Total Devices : 2

             State : clean 
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync


              UUID : ca03d8fc:3d697cfb:9f4aebc1:6f55584c
    Number   Major   Minor   RaidDevice State
       1       8       32        0      active sync   /dev/sdc
       0       8       48        1      active sync   /dev/sdd

Both disks are "idle, normal, clean".