Turns out Linux now requires way more space to run properly than it needed the last time I checked.
- I had 19.7 Gb of free space and based on this Stuck at login screen, that person’s Linux Mint didn’t want to log in without some arbitrary and unknown minimum amount of space.
Long story short, I moved some files from my /home directory to an external drive and I was able to log in, but only after restarting. Now, I have a bunch of errors from random apps and/or operations that say “you can’t do this because you don’t have enough space”.
I now have 47.4 Gb of space… 
Now, let me go look up how to change file and directory ownership because I moved those files with the root account. 
→ Achieved using sudo chown -R username:username /mnt/backup/ in case anyone is interested.