The software is no longer able to log in to the English Wikipedia, following a security fix https://bugzilla.wikimedia.org/show_bug.cgi?id=23076
I don't get it.
Please, try deleting "Cache" subdirectory in your DotNetWikiBot folder. Does it log in without cache? And does it stop logging in when cache appears again?
Updated once again. v2.91
Please, let me know if it works for you.