After restart, all user settings restored to default

[BinaryBlues]

Hi! I originally posted this over at Reddit first, but I figured I might as well try here. As with many of you, my Mac is my livelihood and I'm very dependant on it, so I cannot help but get somewhat uneasy when it fails.

So, my beloved friend and trusted working partner suddenly malfunctioned after 5 years of faithful service.

History:
Earlier this summer my MacBook Pro started behaving weird, closing down apps when I tried to minimize them, opening new documents when I only tried to save, etc. I ran a hardware test and got an error message "4HDD/11/40000000: SATA(0,0)." I feared the SSD was faulty, but when I googled it I noticed that it was a general fault in the hardware test, that all MacBooks from early 2013 got that error message. I made a clean install of High Sierra and reinstalled every app one by one, and it has worked like a charm ever since. Until today that is...

Step by step explanation:

1. Tried to watch YouTube at first but the site would load properly.
   
   
2. I figured it had something to do with the internet so I restarted the Wi-Fi (on Mac) and reconnected my VPN client, but then I got a message from my VPN-client saying 'Killswitch' had been activated.
   
   
3. Tried to open 'CleanMyMac' to flush DNS, but nothing worked...
   
   
4. Restarted my MacBook, and when logging back in, I was asked to confirm permission for Siri
   
   
5. When I finally got in, all my user settings had been reset. All my configurations were gone. I had been logged out of every service I use (Dropbox, Google Drive, iCloud, iMessage, Setapp etc.)
   
   
6. All of my extensions in chrome are removed from the 'Chrome Bar' and chrome claims all extensions may be corrupted (except Grammarly... and I'm also seeing a tonne of ads for Grammarly on Chrome now that my Adblock has been disabled... weird)

All of my apps and files appear to be there though (thankfully!) I ran first aid in Disk Utility but found no problem. I logged back in on all the apps and services and everything appears to work fine now, but I don't trust this Mac anymore, which is a first...

Information:
MacBook Pro (Retina, 15-inch, Early 2013)
2,7 GHz Intel Core i7
16 GB 1600 MHz DDR3
Intel HD Graphics 4000 1536 MB
macOS High Sierra Ver. 10.13.6 (17G65)

What could have happened? And how do I fix this? Very grateful for any help!

// Cheers