Hi, I am the main FBBE developer

I can confirm FBBE works without RPC authentication, only via unauthenticated REST as a design choice.

I am not sure why you were seeing those log messages about authentication attempts but they should not be related to FBBE (or there is a bug I am not yet aware)

> though I suspect it also applies to the REST API as well as I have not seen a different set of credentials for it.

The point of the REST interface is exactly that you don't need authentication, as clearly stated in the title of the official doc https://github.com/bitcoin/bitcoin/blob/e253568da8dc3693ad99aee260276233245e3d77/doc/REST-interface.md
The REST interface provides only data about the blockchain and the mempool, it cannot interact with the wallet, this is why could be used without authentication

I don't know what's the cause of why it isn't working on FBBE without an RPC login but based on the error it requires RPC credentials.

If the BTC RPC explorer worked then why not use that instead of FBBE?
Or I think bring your issue directly to their issue page below the developer might be able to help you
- https://github.com/RCasatta/fbbe/issues

I never heard of this FBBE and I tried to check the GitHub page and codes I can't seems to find where you can edit RPC configuration unlike other Block explorer out there that has .env file where you can configure RPCuser/password.

Why not use another explorer like BTC RPC Explorer? There is a guide here that you can follow.
- https://bitcointalk.org/index.php?topic=5368219.0

What exactly the 3rd part application?
Are you trying to run a blockchain explorer?

The error seems related to RPC since you don't added some flags under bitcoin.conf to run RPC. Copy this setup below and paste it to the bitcoin.conf