  Using novel metallic magnetic calorimeter detectors at the CRYRING@ESR, we
recorded X-ray spectra of stored and electron cooled helium-like uranium
(U$^{90+}$) with an unmatched spectral resolution of close to 90 eV. This
allowed for an accurate determination of the energies of all four components of
the K$\alpha$ transitions in U$^{90+}$. We find good agreement with
state-of-the-art bound-state QED calculations for the strong-field regime. Our
results do not support any systematic deviation between experiment and theory
in helium-like systems, the presence of which was subject of intense debates in
recent years.
