The site was fine, then the hosting provider migrated the accounts from a server to another. After that, there was some issues on the site, which I am not completely sure what exactly, as the client just spoke with the hosting company requesting to have "her site back as it was". After few days, she contacted me.
What was left to do was the taxonomy display (driven by view) that displayed back in the default settings. I've checked the view was active (which it was) so then I just cleared the cache and everything went back to normal. The issue though is that every now and then, it reverse automatically back to the default view - and clearing the cache again does the trick.
I have no idea what is the trigger behind, sometimes happens after few hours, sometimes after days.
If anyone has a clue, any suggestions/thoughts are welcome!