If you still have access to it, "/system reset-configuration". If you don't, NetInstall with "keep configuration" unchecked. That's covered by the wiki.
It should restore the factory config it came with.
When do you get the restore error? Do you click the "Restore" button in the file list? Are you sure you have the right backup file selected when clicking that button?
Anyway, meanwhile, I've reseted the router as you said above, and it did just what I needed. Thank you very much.
I've reconfigured it again, and I'm close to making the whole thing run.
How do I increase TTL by 1?
The ISP sends the packets with TTL=1, in order to block some entry level routers.
I have the same problem. But what I did to test was to do a brand new backup and immediately try restore that backup. It still gives me the error "Couldn't restore configuration - no file name (5). Does that throw the theory out that it might be incompatible versions?