Since last month,[1] action=import would ignore the rootpage parameter when
the namespace parameter was specified, due to underlying changes in
MediaWiki to make Special:Import clearer. Starting with 1.26wmf7,[2][3] it
will instead give an error if both parameters are specified to avoid
incorrect importing by code expecting the old behavior.
[1]:
https://gerrit.wikimedia.org/r/#/c/179116/
[2]:
https://gerrit.wikimedia.org/r/#/c/210945/
[3]: See
https://www.mediawiki.org/wiki/MediaWiki_1.26/Roadmap for the
deployment schedule.
--
Brad Jorsch (Anomie)
Software Engineer
Wikimedia Foundation