Terminal not working on Yosemite Not sure about the Yosemite version because "About this mac" option doesn't work too. Terminal, X11, Screenshot to name a few, don't work.

Used to work fine since I updated from Lion a few months ago.

This message shows up in "console" when I try to open one of those options: 


  23/1/18 21:36:52,032 com.apple.xpc.launchd[1]:
  (com.apple.Terminal.11392[853]) Service exited with abnormal code: 1


I found this answer from 2014:


  You installed another version of bash,right? The default login shell
  is /bin/bash. you can change it following these steps,
  
  go to "System Preferences" > "Users &amp; Groups" click the "padlock" icon
  and authenticate right-click the icon for your user and select
  "Advanced Options..." change the value for "Login shell"


My default login shell is "/bin/bash" so I assume that this user didn't had the same problem after update to Yosemite.

At this point I'm completely lost with my poor knowledge on IT...

Thanks in advance!
Troubleshooting is a process of elimination and often requires some patience. In your case the best option may be to just reinstall macOS, but before doing that there's a few things we can try.

The first thing I'd probably do is start your Mac in Safe Mode and test to see if you can use your system 

Boot into Safe Mode

Follow these steps to boot your Mac into Safe Mode:


Fully shut down your Mac
Restart your Mac
Immediately press the Shift key and keep it down 
Let go of the Shift key when you see the login window (NOTE: If you have FileVault enabled you may need to log in twice).
Take a note of what happens (i.e. can you launch Terminal, view About this Mac, etc)
Exit Safe Mode by restarting your Mac as normal
Again, take a note of what happens (i.e. can you launch Terminal, view About this Mac, etc)


Once you've booted into Safe Mode, let me know how you went and we'll go from there. Primarily we need to establish if everything works okay in Safe Mode or not, and whether things get back to normal afterwards.