MBP running El Capitan freezes at login
On Apple Support Communities I got a helpful answer I will copy here for others. I don't exactly understand what it does exactly.


  Log in as Guest. 
  
  This procedure will delete certain temporary and
  cache files. The files are automatically generated and don't contain
  any of your data. Occasionally they can become corrupt and cause
  problems such as yours. 
  
  Please back up all data. 
  
  Triple-click anywhere in the line below on this page to select it: 
  
  /var/folders
  
  Right-click or control-click the highlighted line and select Services ▹ Open from the contextual menu.* 
  
  A folder with the odd name "folders" should open. Inside "folders" are several subfolders, each with a two-character name. Drag all the subfolders
  except the one named "zz" to the Trash. Don't delete the subfolder
  named "zz". You'll be prompted for your administrator login
  credentials. Restart the computer and empty the Trash.
  
  * If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination Cmd-C. In the
  Finder, select
            Go ▹ Go to Folder... from the menu bar and paste into the box that opens by pressing command-V. You may not see what you pasted
  because a line break is included. Press return.


I have to mention that for now, it solved my primary issue. However, I'm experiencing some extraordinary slow speed on every app up to now. I guess that this might be caused by the missing temporary files that the system is rebuilding.

Hope this helps somebody!