I had the same problem. It persisted despite clearing the website cache and reloading the page.
My Fix:
I have no explanation why this worked but when I clicked on the Sign In button at the top right of this help page, I was redirected to a new tab that allowed me to successfully log in to ChatGPT without the error. The URL is identical to the general ChatGPT login so I can’t explain why this worked.
I am using Firefox ver. 128.9.0esr + Linux