JorgeB

Replace the device. 

This.

OK, now with array stopped, unassign all devices from that pool, start array, stop array, re-assign all 4 pool devices, start array, post new diags if it doesn't mount.

Filesystem Size Used Avail Use% Mounted on rootfs 16G 16G 22M 100% /  
Rootfs is full, this usually happens if you have some mappings to anything other than /mnt/user, since they will be mapped to RAM, check all.

Try switching to ipvlan (Settings -> Docker Settings -> Docker custom network type -> ipvlan (advanced view must be enabled, top right))

Disable docker service, click on the appdata share, change to cache=prefer and run the mover.

(path: appdata/Nginx-Proxy-Manager-Official/data/logs/proxy-host-1_access.log) (path: appdata/Nginx-Proxy-Manager-Official/data/logs/proxy-host-2_access.log)  
The corrupt files are listed in the syslog, like the examples above, these need to be deleted/restored from backup, then run another scrub to confirm there are no more uncorrectable errors, also the Lexar SSD appears to be failing, probably a good idea to replace it.
 

If you are using Firefox reboot first then use a different browser.

You have to restart the array in normal mode.

You can add (not replace) one of the 1TB devices, wait for the pool to balance, when done you can replace the 250GB with the other 1TB.

SMART looks OK and it's not logged as a disk problem, check/replace cables, since the emulated disk is mounting and assuming contents look correct you can then rebuild on top.

Probably for the best.

If eth2 is now the main NIC you must set it eth0, bottom of network settings page, then reboot.

Disk dropped offline, but it looks healthy, and there are UDMA CRC errors logged, so replace the SATA cable and try again.

Try using a new flash drive with a stock Unraid install, no key needed, if it still doesn't work the problem is not the software, could be NIC or some other LAN issue, if it works recreate the Unraid install.

Flash drive problems:
 
Mar 20 19:37:27 Valentine kernel: FAT-fs (sda1): FAT read failed (blocknr 1840)  
Try running chkdsk on it, or recreating it.

I think you should just reformat and restore from the backup, something weird happened there.

You could have separated the output by disk better but unless I'm missing something no GPT errors detected, you'll need to connect one disk at a time to see which one the board is having issues with.

Just format the disks, parity will remain valid, stop array, change fs to a different one for all disks, start array, format, stop array, change back to xfs, start array, format again.

You should not rebuild, you should do a new config (Tools -> New config) with the cloned disk3 installed in place of the old one.

Nope, and 7.5h sounds about right for the model and capacity of the disks involved, usually count with 2 to 3h per TB.

Try rebooting.

This is likely an incompatible plugin, reboot in safe mode to confirm.

Mar 13 02:04:47 NAS kernel: ata3: link is slow to respond, please be patient (ready=0) Mar 13 02:04:51 NAS kernel: ata3: SATA link down (SStatus 0 SControl 300) Mar 13 02:04:51 NAS kernel: ata3.00: disable device Mar 13 02:04:51 NAS kernel: sd 3:0:0:0: rejecting I/O to offline device  
Disk dropped offline, this is usually a power/connection problem, check/replace cables, both power and SATA.

Remove the gateway from eth2, only eth0 should have a gateway set.