@Antzi's answer links to the recent Ars Technica article NASA’s EM-drive is a magnetic WTF-thruster; Test reveals that the magic space unicorns pushing the EM-drive are magnetic fields.. According to that article, results of a study of the proposed, "observed", and "measured" Em Drive phenomenon were recently presented at Space Propulsion 2018 by a group from the Technische Universität Dresden's Institute of Aerospace Engineering. The results indicate that the previous observations were wrong, and go on to address why.
If I understand the highly stylized article, the magnetic shielding of the earlier experiments was inadequate, and allowed some field to penetrate. This then interacted with DC current running in power leads. The RF amplifier uses ampere-scale (probably many) DC current and was a likely suspect, and discussed in footnote 3 of the original paper. I'd proposed a similar problem in this question but I'd assumed they'd competently shielded the Earth's field and so asked about the magnetic field produced by the current itself.
But at this point I'm just working off of the Ars Technica article.
Are there any technical write-ups of the Dresden work available? Slides of the presentation perhaps, or a video of it?