Could somebody put more details on this? We are not mining on any old blocks for sure.

Maybe your miner considers a block as old if we switch back to a previously mined coin? Consider this:

1) We are mining block X of coin A.
2) Coin B just lowered difficulty and we switched to coin B as it became more profitable and we're mining block Y.
3) We mined block Y of coin B and its difficulty rose again so it stopped being profitable.
4) We switch again to coin A, but since it didn't have any blocks found in the meantime, we are mining on the same block again, so we try to hash again block X.

Could some mining software thing that this is an old block just because it was working on it previously, then switched to another block and then switched to the same previous block again? In single-coin mining this scenario (getting again a previously mined block) means that the pool sends an old block. But in coin-switching this is perfectly valid and your miner should continue mining on the block X without issues.