I had a similar problem, too. The solution that worked for me is tricky. Here is the detail of what I have done. FYI.
I have a SanDisk Cruzer Micro 2GB UsbStick running unRAID 6.9.2 on Asus P7P55D-E LX machine. CSM boot, UEFI. turned off. Nothing wrong.
Then I tried to use the same UsbStick on Gigabyte GA-MA78GM-S2H machine. This old machine only has BIOS. I can boot to unRAID, both command UI and Graphic UI, but I can't connect to unRAID WUI either from localhost or remte PC. At first, I thought it was the problem of NIC, maybe driver or DHCP. I tried to fix it but failed.
In this thread, the solution suggested seems to be related with BIOS settings, but that does not work for me. Then I found the path "/boot" was not mounted on my unRAID machine. It's the real cause of the problem.
I have a Kingston DataTreveler 32GB UsbStick. I tried to use "Unraid.USB.Creator.Win32-1.6.exe" to create unRAID on the UsBStick, but the process stuck at syncing files, never finished. I had to close the application manually.
Finally, I used Rufus to dump the img (vhd) from SanDisk Cruzer Micro 2GB UsbStick, and wrote the img to Kingston DataTreveler 32GB UsbStick. The partition is 2GB only, left 28GB around unoccupied. This time, Kingston DataTreveler 32GB UsbStick works on Gigabyte GA-MA78GM-S2H machine, and /boot is mounted. Everything works fine, for now.
I can't explain why /boot is not mounted with SanDisk Cruzer Micro 2GB UsbStick, while same data on Kingston DataTreveler 32GB UsbStick just work on the same Gigabyte GA-MA78GM-S2H machine. If anyone has an explanation, please share. Thanks.