JorgeB

If it's just a log problem this can help:
btrfs rescue zero-log /dev/sdc1 If it works make sure backups are up to date since this problem has been known to reoccur, if it does suggest backing up and re-formatting the pool.

Recommend the 2nd option, see below for more details:
https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?do=findComment&comment=511923
 

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, the problem is usually not just about not enough RAM but more about fragmented RAM, 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/

Do this:
https://wiki.unraid.net/Manual/Troubleshooting#Lost_root_Password

Parity check won't restore any data, based on the btrfs transid both docker and libvirt images are new, I see you have an unassigned SSD, maybe that was your cache drive before? And the old libvirt and docker images are there?

If it's a pool it will only format the pool, next to the format button there will be a list of the devices that will be formatted.

Yes.

Reboot, but there's corruption on the cache device, post new diags after the reboot and array start.

Run it again with the --rebuild-tree option.

The only issue I see logged is about the Nvidia driver, does it help if you temporarily uninstall the driver?

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.