Internet Accounts Unknown Error

Device information: MacBook Pro 13" Mid-2012, using macOS Catalina 10.15.7

Problem:

It is impossible to login to any internet account (Googe, ICloud, etc.), since an "Unknown Error" pops up after introducing each account's email and password. Furthermore, when trying to add a new internet account from system preferences, absolutely nothing happens: it is like clicking buttons without an assigned action.

I have reasons to believe that this is due to a possible preference files deletion, by mistake, of course. Thing is, I do not see anyone having this exact same problem in any forum. Help is very much appreciated.

Notes:

• Trying to add an account via "Mail", "AppStore" or similar system-default applications causes the same error.
• Accessing accounts from a browser works flawlessly, Safari included.
• "Airmail 4" works too, yet I guess it is due to the fact that it does not "add" an account; it is just an access point.
• Any kind of security permissions that might be involved in the setup of a new account are not a problem, since the user I use is part of privileged groups.
• System checks via "EtreCheck" do not report any issues related to system preferences.
• Restarting the device does not solve the problem.
• Updating macOS is not viable.
• Reinstalling macOS is a worst-case-scenario solution.