Bitcoin Core v0.21 RPC credentials going bad
Ok, I managed to find the culprit. It's actually an application I have running in the background that I simply completely forgot about it and it is trying to access to the RPC interface and as I have never paid too much attention to it, I think I never setup anything related to RPC in it. So, most probably that's the problem.
I used netstat -p -at and ps -ef to figure out what exactly was interacting with bitcoin core daemon.
Thanks every one. I think this one is solved.
Psy