I have the same error message.

/usr/lib/lua/luci/dispatcher.lua:433: Failed to execute cbi dispatcher target for entry ‘/admin/system/fstab’.
The called action terminated with an exception:
/usr/lib/lua/luci/cbi.lua:311: Unable to read UCI data: fstab
stack traceback:
[C]: in function ‘assert’
/usr/lib/lua/luci/dispatcher.lua:433: in function ‘dispatch’
/usr/lib/lua/luci/dispatcher.lua:168: in function </usr/lib/lua/luci/dispatcher.lua:167>

I noticed that we both have a blank space in our filesystem labels and mount shows:

/dev/sda1 on /mnt/My\040Passport type ext4 (rw,sync,relatime,data=ordered)

Maybe this is the reason why the fstab can’t be created in luci.