JorgeB

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

Check filesystem on disks 1 and 12.

Check that the flash drive is being detected and set as the 1st boot option.

Make sure this is done and if yes enable the syslog server and post that after a crash.

Controllers with a SATA port multiplier are not recommended in general, but that by itself should not cause syn errors, unless it's a faulty controller, they are usually no good for performance and reliability issues.

One of you cache devices dropped offline in the past, run a correcting scrub and check that there are no uncorrectable errors, also see here for how to reset the errors and better pool monitoring.

Then looks like a compatibility issue between the NIC and that server, if it's not detected on a hardware level it will never work.

Stop array, click on that disk, change filesystem to xfs, start array, format.

Yes and yes.

Start the array in maintenance mode and post the output of
xfs_repair -v /dev/md4 and
xfs_repair -v /dev/md5  

Then you have other issues, a stock install comes without network.cfg, it's only created after the user makes any change to the network settings, try booting with a stock Unraid install using a different flash drive, no key needed, just to see if it's booting.

If it's a one time thing you can ignore, if it keeps happening try limiting more the RAM for VMs and/or docker containers, alternatively a small swap file on disk might help, you can use the swapfile plugin:
 
https://forums.unraid.net/topic/109342-plugin-swapfile-for-691/

You need to increase the shutdown timeout, try 120 or 150 secs. (Settings -> Disk Settings)

For now the only issue is with loop2, that's the docker image, delete and re-create.

Like mentioned if it crashes during just the file copy it suggests a hardware problem, could ne PSU, board, etc.

Please use the existing plugin support thread:
 
 

It should work correctly with any browser except Firefox, it should also work with Firefox if you don't it "Resend" when asked.

Some sync errors are normal, even expected after an unclean shutdown, just run a correcting check.

UDMA CRC are usually a bad SATA cables, the other ones are more concerning, it means the drive reallocated sectors, and with those drives that can cause data corruption, possibly due to bad firmware.

With reiser this can take a long time, more than 1 hour, if the disk was not cleanly unmounted, suggest converting to XFS ASAP

Unassign both pool devices, start array, stop array, re-assign both devices to their new positions, start array.

SSDs don't need much power, could be a bad power connection or bad PSU, SATA cables/controller are also an option.

Yes, just run xfs_repair using the GUI, without -n or nothing will be done and if it asks for -L use it.

That's good, see here for how to reset stats for cachetwo, then see also there how to better monitor the pools, if more corruption errors appear you likely still have some hardware issue.

That suggests the backup is no good or incomplete, see if you can copy the bare minimum, like your key, super.dat, pools folder and the docker templates, then reconfigure the rest.