Edit: I figured it out. It was caused by the User-Agent Switcher and Manager addon, I use it to switch user agents on one site. I switched it to whitelist mode in the settings and now it works.
Like when I try to log into chat.openai.com, I click the checkbox that I’m human and it just keeps reloading that checkbox without progressing the page. But when I open the tab in a Firefox container (which you can install with the Firefox Multi-Account container addon), it works.
I want it to work without a container open.
That could depend on the VPN, no? If you pick a server somewhere sketchy, you could be more likely to be flagged.