  For more than 40 years, most astrophysical observations and laboratory
studies of two key soft x-ray diagnostic $2p-3d$ transitions, $3C$ and $3D$, in
Fe XVII ions found oscillator strength ratios $f(3C)/f(3D)$ disagreeing with
theory, but uncertainties had precluded definitive statements on this much
studied conundrum. Here, we resonantly excite these lines using synchrotron
radiation at PETRA III, and reach, at a millionfold lower photon intensities, a
10 times higher spectral resolution, and 3 times smaller uncertainty than
earlier work. Our final result of $f(3C)/f(3D) = 3.09(8)(6)$ supports many of
the earlier clean astrophysical and laboratory observations, while departing by
five sigmas from our own newest large-scale ab initio calculations, and
excluding all proposed explanations, including those invoking nonlinear effects
and population transfers.
