How can this happen?

Did you had to relocate any files?

Have you changed anything with your tracks and have they been analyzed?

Yes I had to relocate some files and no, I didn't changed anything with my tracks (they are still in the same folders on the same drive).

You've somehow screwed up the database - it uses unique identifiers to match database entries to the files themselves. How many songs are in your collection?

I have over 4k songs. I think the best thing is to totally uninstall Rekordbox right? Would be a pain since I've just finished putting it back up again after I have a new PC. If so, can you give me a guide to how best do that?

Thanks in advance

No - uninstalling doesn't remove the database itself. You can keep the installation.

You say you recently "put it back up again after I have a new PC" -- did all of this start after you moved your collection and music to a new computer?

Yes it did. I build my own PC and moved windows to an SSD (so that's my new startup disc C:/) and took out the hard drive from my old PC (with all my music and rekordbox stuff) to become a store drive which is in my E:/ slot now.

It's a mystery to me: when I open the song "thrift shop" (for instance) with VLC or Itunes.. it plays the song perfectly well, but when I open it whit rekordbox it plays a whole different song.

I must have messed something up (badly), but I'm not sure what could couse this to happen

Side note: I just found an old Rekordbox library save file (EDB) from 2014, I could try to rollback to that..?