That's because the algo isn't NIST5.
It's a fork of it, without any name. The original person who made the coin you forked from, made it all wrong.
Maybe and may not be, it has more than 5000+ blocks already. Mining works file through wallets, it's just the pool that's causing the problem.
Yes that's because mined all current blocks from wallet. And mining from the wallet is like mining on the actual algo inside source code, whatever algo it is.
Any coin can be mined from it's original wallet whatever algo it is.
But mining with a pool, it's mining using a stratum. And the stratum requires the algo to be installed in it to properly work.