Try this: in your screenshot, to the left of "Not secure" (ie. in the upper left corner, to the left of the URL), do you see that little "i" in a circle? Click it.
Then click "Cookies". You should see a dialog with a list of domains (I have "project1999.com" and "wiki.project1999.com"), and a "Remove" button at the bottom. Click the "wiki.project1999.com" entry to highlight it, then click remove.
Refresh the page, and try to login again. Hopefully that will fix it. If not, do the same thing, but this time also get rid of the "project1999.com" one. If that
still doesn't work ... post back
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