Thanks for the reply, but no joy, unfortunately. I linked to the contract in my question, and the contract address is indeed the one I can see in .openzeppelin/rinkeby.json. So, this is the command I used:
npx truffle run verify Foo@0xdb170C5c84f2228425B1fbEc9B121AD77D8ef72D --network rinkeby
Still the same problem. I created an issue on GitHub about it, also. And it seems someone else also has the same problem.