Can't log in to panel


It seems like I’m having the same issue as the folks on this thread.
I didn’t change anything but the panel won’t log me in (or other users). The site is hosted on AWS LightSail, running 3.2.3 on php ver 7.1.28.

I also looked at the other thread. Removing session files didn’t help (there were about 20 session files), and it looks like cookies are not the issue either. What’d be the best way to move forward from here?