Unable to log-in
You definitely don't need to make multiple accounts, one is enough to interact with the entire fediverse.
The only issue is your one still runs Lemmy 0.17.4, and it sounds like the app expects the instance to be on 0.18.0.
This won't always be the case, lemmy.world will eventually upgrade their Lemmy version and you will be able to use your account with them on the app. If you don't want to wait for them to upgrade, then making another account is an option.