Unexpected Error with Login Session - haven't been able to login for weeks

Since the start of preseason, I haven’t been able to login any of my accounts (have one each in NA, EUW, JP, OCE to play with friends) so it’s definitely not an account problem. LoR and Valorant works fine for me.

When I log in, the game appears to be loading forever, with my icon being default and friend list being empty.

Here’s the screenshot after a fresh install.

After maybe 2 minutes, I get the popup "There was an unexpected error with your login session." and the only option is to Exit the game.

Here’s a list of things I have tried:

• Everything on the official support page
• Uninstalling → Delete Riot Games folder → restart → installing twice
• Create a new account and login
• Using a VPN as recommended by
  some reddit threads
• Changing locale to match server
• Changing timezone to match server
• Clearing my DNS cache
• Tethering to my mobile network
• Opening a support ticket

I do not want to reformat my computer as it is my workstation and it would take days to set it up back to its current state.

Here’s my specs for reference:
Windows 10 Pro 64-bit, 64 GB RAM, Ryzen 3950x, RTX 2070 Super