Chrome since version 80 and Firefox since version 60 introduced a cookie privacy feature, "SameSite", which in short means that the authentication cookie WikiDot uses does not work cross-domain from "wikidot.com" to "scmapdb.com".
tl;dr: Temporarily use EDGE to work around this problem. (Other browsers may also work, not got Safari to test that.)
TOR will also fail as that's based on Firefox.
More information here: http://scp-wiki.wikidot.com/third-party-cookies
We could work around this issue at a site level by temporarily reversing the redirect from "scmapdb.wikidot.com" to "scmapdb.com", so that we enforce NOT using our custom domain name, or just temporarily disable the redirect.
Otherwise this issue needs to be pressured on WikiDot, as after all this site is a paid service. WikiDot have created an issue for this, but annoyingly left it at medium priority: https://scuttle.atlassian.net/browse/WDBUGS-9
This should be a serious priority… Also annoying that nobody outside WikiDot is allowed to comment on the issue. We can do so here though: http://community.wikidot.com/forum/t-13660089/i-can-t-login-scpwiki
(I've put my thoughts there given this is a Pro Plus subscription. Not an acceptable time scale!)
Thanks,
Adam Reece.