Slushpool might be old and no updates anymore. 4 years ago, Slushpool and all of their contents were moved to Braiins. If the Braiins pool works for you, then better use Braiins pool since it's also owned by Slushpool. There's no problem with using the Braiins pool.

I dig into their guides, and it seems that they have stratum v1 and v2. Would you mind sharing with us the slushpool stratum link that you use? Maybe the slushpool link you use is currently not supported based on their guide below the stratum v2 doesn't support bitmain stock firmware. So possible, that is the reason why it doesn't work.

You can find the guide here: https://academy.braiins.com/en/braiins-pool/btc-mining-setup/