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.

 [3]: See for the deployment schedule.

Brad Jorsch (Anomie)
Software Engineer
Wikimedia Foundation