I was able to login fine with my personal account (linked to dev account), but any other account throws this error.
Any ideas? Thanks!
Haven't seen this error before. From "if you are using software which hides the origin of the request" , are you using a VPN or proxy by chance? Maybe try turning it off to test.
Otherwise could be worth filing a report to https://developers.facebook.com/support/bugs/
Not using VPN, but thanks for suggestion.