ReadyNAS in Lifesupport

Please don't post logs publicly - there is some privacy leakage when you do that. I've redacted your link.

The most important thing to do now is back up all your data, since it is certainly at risk.   You might not be able to get all of it.

In parallel, you send a private message to one of the mods ( perhaps JohnCM_S ) with the link to your logs, and ask him to analyze them.  You send a PM by clicking on the envelope icon in the upper right of the forum.

FWIW, I did take the liberty of taking a quick look at your logs. There are problems.  JohnCM_S  might see things that I've missed.

System.log:

May 17 08:54:37 Jergens-NAS kernel: end_request: I/O error, dev sdd, sector 3352498240
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343060984 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: Disk failure on sdd5, disabling device.
May 17 08:54:37 Jergens-NAS kernel: <1>md/raid:md2: Operation continuing on 2 devices.
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343060992 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061000 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061008 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061016 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061024 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061032 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061040 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061048 on sdd5).
May 17 08:54:37 Jergens-NAS kernel: md/raid:md2: read error not correctable (sector 3343061056 on sdd5).
...
May 17 09:02:37 Jergens-NAS RAIDiator: Detected increasing pending sector count[906] on disk 4 [WDC WD20EARS-00MVWB0, WD-WCAZA4957797].
...
May 17 09:02:55 Jergens-NAS RAIDiator: RAID sync finished on volume C. The array is still in degraded mode, however. This can be caused by a disk sync failure or failed disks in a multi-parity disk array.
May 17 09:02:56 Jergens-NAS RAIDiator: RAID event detected. (Jergens-NAS) : RAID sync finished on volume C. The array is still in degraded mode, however. This can be caused by a disk sync failure or failed disks in a multi-parity disk array.\n\n[Fri May 17 09:02:52 EDT 2019]

So disk 4 (sdd) has failed (over 900 pending sectors).

I also see disk 2 (sdb) marked as a spare, so it's not clear to me why you still have access to your data.

mdstat.log

Personalities : [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
md2 : active raid5 sda5[4] sdd5[3](F) sdc5[5] sdb5[6](S)
      5846379648 blocks super 1.2 level 5, 64k chunk, algorithm 2 [4/2] [U_U_]
      
md1 : active raid6 sda2[4] sdd2[3] sdc2[5] sdb2[6]
      1048448 blocks super 1.2 level 6, 64k chunk, algorithm 2 [4/4] [UUUU]
      
md0 : active raid1 sdb1[6] sda1[4] sdd1[3] sdc1[5]
      4194292 blocks super 1.2 [4/4] [UUUU]
      
unused devices: <none>

Whatever's going on, I don't think you should reboot the NAS until after you've backed it up.