I had the problem but I figured it out.
So my issue is that when I'm logged out on mobile, site works fine. The moment I log in I'm greeted with 500 error. On PC I can log in fine.
The problem lies in Opera Mobile, and in particular the Data Saving toggle. Disabling it fixes the problem for me.
That being said, I like that toggle, and it'd be great if you could look into what's causing this. Might be some interaction with a DDoS/malicious login prevention plugin.