Just to keep everyone updated, i discovered what the issue really was. Before all this happened I fidgeted with several plugins allowing to crosspost from/to Tumblr. These were very outdated plugins but reportedly still working, what I did was trying them live (and this is very wrong I know) and then, since for some reasons they were not working due to the Tumblr API changing like there’s no tomorrow and Tumblr posts formats and the like, I removed them. What I did not consider is these plugins were doing something I was not aware of to the way CP handles post types to make CP post types able to be posted in Tumblr as Tumblr post formats.
Fact is removing the plugin left somewhere in the code something that affected the whole site.
What sparked understanding is @anon71687268 remark about which plugins were calling the functions, and @joyously pointing me towards detecting bad code, but also the question about deleting plugins via ftp or messing with database.
Now the question I have is: How does CP is going to handle plugins leaving stuff behind them? This experience leads me to think that a plugin that leaves a reminder is no good to a site. I guess I should just open a thread for that…