From during the bootup sequence:
Jun 25 19:55:14 Tower-NAS kernel: FAT-fs (sda1): error, clusters badly computed (450 != 449)
In theory, a reboot *should* fix you up, but for the best success, shut down the server, pull the flash and toss it into a Windows / Mac box and run it's file system checks on it (Win: right click on drive, properties, tools, Check) and then try the server again.
Worst case scenario, replace the flash https://wiki.unraid.net/Manual/Changing_The_Flash_Device