From the looks of it, it's due to the recent MediaWiki update to Wikia/Fandom, specifically on Wikis that have already been "migrated". Attempting to log into these results in a "Login Failed" message. This applies if my site preferences are for 'Fandom' as well as an HTTPS 'custom', if I log in via my saved profiles as well as re-entering the correct details.

As this issue only affects wikis that have already migrated to the new MediaWiki software (e.g. galactica.fandom.com) and works fine on non-migrated wikis using the older software (e.g. residentevil.fandom.com), this would seem to be a widespread compatibility problem.

I am presently using:
AWB Version 6.0.6.1.
Internet Explorer version: 11.0.19041.508
.NET version: 4.0.30319.42000
Windows version: 6.2