"block decode failed" error while mining in pool
Are you running "bitcoin-cli submitblock xyz" manually? You are not supposed to do this. Software does this. If you mine in a pool you don't even need to run bitcoind at all.

If you do want to run your own bitcoind node, don't mess with the blk*.dat files. You don't need to touch them. The bitcoind program is the only thing that needs to use them.

If you mine in a pool just set up your ASIC mining machines and make sure they are running, then use the pool website for the rest of your needs.