Aha! Simple is right. For the record:
The login issue with the new versions of MediaWiki has to do with the addition of a cookie prefix, which the new Scripto plugin takes into account.
https://github.com/omeka/plugin-Scripto/issues/28
@jimsafley 2017-15-06
Yes, thanks to @mebrett’s patience, we’ve found that the cookie prefix is predictably in one of the following formats:
<dbname>if a table prefix is not used, -or-
<dbname>_<tableprefix>_if a table prefix is used
<dbname>is the MediaWiki database name and<tableprefix>is the MediaWiki table prefix, in case one is being used. (Note the trailing underscore in the second format.)
(sigh)