So step 1, revert the reshape. Step 2, get the array back running. 
Step 3, start the reshape again.

root@nas:~# mdadm --readwrite /dev/md0
root@nas:~# mdadm -D /dev/md0
/dev/md0:
            Version : 1.2
      Creation Time : Sun May 17 00:23:42 2020
         Raid Level : raid5
         Array Size : 35156256768 (33527.62 GiB 36000.01 GB)
      Used Dev Size : 11718752256 (11175.87 GiB 12000.00 GB)
       Raid Devices : 4
      Total Devices : 5
        Persistence : Superblock is persistent

      Intent Bitmap : Internal

        Update Time : Tue May 26 01:13:08 2020
              State : clean
     Active Devices : 4
    Working Devices : 5
     Failed Devices : 0
      Spare Devices : 1

             Layout : left-symmetric
         Chunk Size : 512K

Consistency Policy : bitmap

               Name : nas:0  (local to host nas)
               UUID : d7d800b3:d203ff93:9cc2149a:804a1b97
             Events : 38605

     Number   Major   Minor   RaidDevice State
        0       8        1        0      active sync   /dev/sda1
        1       8       33        1      active sync   /dev/sdc1
        2       8       49        2      active sync   /dev/sdd1
        4       8       65        3      active sync   /dev/sde1

        5       8       81        -      spare   /dev/sdf1
root@nas:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] 
[raid4] [raid10]
md0 : active raid5 sda1[0] sdf1[5](S) sde1[4] sdd1[2] sdc1[1]
       35156256768 blocks super 1.2 level 5, 512k chunk, algorithm 2 
[4/4] [UUUU]
       bitmap: 0/88 pages [0KB], 65536KB chunk

unused devices: <none>

=================================

ok, got a bit too scared :)

it´s now working again. so I can plugin a UPS and restart grow.

guys, thank you all very much for your help.