So, it looks like I am running in to this same issue as well.. The best I can describe it for you is:
On the first time of testing, clicking the facebook button, opened up a pop up to sign in/give permission from facebook account, it did this on google chrome and edge browser. But i would just X out of the popup, on the last test before I got this error, clicked on facebook button, popup happen, proceeded through to allow FB permissions, but I then clicked the cancel button from Facebook, it loaded for a bit, kicked me back to my website, with this error. (also if you have "osclass pay" plugin by you) it will say "As a bonus for joining website, we have boosted your wallet with $5.00. Feel free to use them for promoting your listings!" right below is the same as above "This account does not exist."
(Also note, the time i canceled on the final fb page, was using the same email on the user account and facebook page, so it would have been connecting the accounts, rather than actually creating one)
However, now when I click the facebook button on chrome it loads for a moment, then kicks right back to the osclass and not exist errors on front. So there is no popup anymore like before. On Edge, it just opens the facebook page to long, not in a pop up, but entire website changes to facebook. (where there was a pop up before)
I see no error logs showing this error anywhere on the website.
There is also a email getting fired out each time it happens, it is the admin-new-user email and all the variable are blank inside the email.
Name: (blank)
E-mail: (blank)
doesn't look as its filling the database with blank/nulled, so thats good.
I have tried uninstalling - reinstalling, clearing caches, but it just instantly is stuck in this mode.
I also uninstalled and installed the other plug-in I didn't take it to the end, but the facebook popup came right back. uninstalled that, reinstalled yours and still nothing.
Moved it from test server to production server also, still same.