Can reproduce, with an account that has pre-Jan-26 cookies (that was when the fix for T124620 was deployed) but not with a fresh one. You can get around this by deleting cookies (or logging out and back). So this is probably fallout from the old bug (same cookie set with different value for wikisource.org and .wikisource.org).
Given that cookies last 30 days, this bug will go away by the end of the week. Wontfix?