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.