I just wanted to add my experience in case it helps someone who has stumbled upon this conversation as I was also seeing this error when attempting to log into FB Messenger on my iPhone and iPad, yet I could log into my FB account without an issue. On my iPhone and iPad, I have the ‘Lockdown’ app installed which enhances blocking of trackers. (You have to enable it in Safari settings/Content blockers.)
I opened the Lockdown app and switched off the Firewall and then found I could once again log into the Messenger app without a problem.
Once logged in, I then immediately switched the Lockdown Firewall back on, but it’s so quick and simple to action that I won’t mind going through that brief ‘off and back on’ process when opening the Messenger app in future if needs must.
So, what I am suggesting is that if you have Lockdown or an alternative Firewall app on your device, try temporarily disabling it as this may well be the solution.
Just an update to my previous post; I actually found that having logged successfully into the FB Messenger app on my iPhone and/or iPad after having temporarily disabled my 'Lockdown' app Firewall, I later happily discovered that I did not have to disable it again prior to opening the FB Messenger app next time, so, at the time of posting, it looks as though the app is now behaving itself without having to touch my Firewall app. Hope this helps.