I had the same issue for twice now (using latest Nightly).
Miranda unable to connect fo Facebook and Facebook temporary locked my profile.
I am not doing much at all on Facebook, but what I have noticed is... When I was active on Facebook, it locked me afterwards. Meaning, the first time, I liked a page and a few moments later, I was locked (Miranda was active in background). So when I enabled my profile back, it asked me "if it was me, who liked this page".
Then it worked fine again, for about 2-3 days.
Then I got a friendship request and I agreed to it. Few moments later, I was locked out again and was asked "if it was me, who agreed to that friend". Miranda was active in background. So maybe, there is a relation to that. In my Miranda settings, I don't have "Show wall posts (news feed)" enabled.