Hi there. I've just had exactly the same issue, after upgrading from 7.02.?? I think? I also upgraded the modem firmware but I dont think that will be related.
I tried the UCI commands, but the FOTA remains OFF and any attempt to turn it on returns the same error?
I then did a Factory Reset, and on reboot the firmware page no longer had the 'FOTA Configuration' tab at all, and when I tried to select 'update from: server' the 'server' was greyed out (could this be because it was already on the latest firmware, so there was no update?).
I then reloaded the previously saved config, and the 'FOTA Configuration' tab returned, and couldnt be turned on.
I have another X11 on firmware 7.04.1. It has the FOTA Configuration switch also OFF, and if I try to turn it on, I get the same error mentioned above.
However, on the 'Update Firmware' page there is an update to 7.04.2 available on the server, and I CAN select 'server' in the 'Update from' area, and then update the firmware, so thats working. It appears to just be an issue with the 'FOTA Configuration' switch, which doesnt appear to stop the updates anyway (at least from 7.04.1).