This article presents an experiment designed to test higher-order quantum electrodynamics (QED) effects and electron-electron interactions in highly charged uranium ions. The experiment uses a multi-reference method based on Doppler-tuned X-ray emission from stored relativistic uranium ions with different charge states. The energy of the 1s1/22p3/2J = 2 → 1s1/22s1/2J = 1 intrashell transition in the heaviest two-electron ion (U90+) is measured with an accuracy of 37 ppm. By comparing uranium ions with different numbers of bound electrons, the experiment disentangles and tests separately one-electron higher-order QED effects and bound electron-electron interaction terms without the uncertainty related to the nuclear radius. The experimental results provide a significant benchmark for calculations in strong electromagnetic fields and can discriminate between various theoretical approaches. The experiment is performed at the Experimental Storage Ring (ESR) at GSI in Darmstadt, where a beam of H-like uranium ions is stored, cooled, and decelerated to an energy of 41.03 MeV/u. The X-ray transitions are detected using two high-resolution crystal spectrometers placed at observation angles of θ = ±90° near the gas-target chamber. The energy differences between intrashell transitions in He-like, Li-like, and Be-like uranium ions are obtained, allowing for the separation of one-electron and many-electron QED effects. The final value of the He-like U transition energy is EHe = 4.509,763 ± 0.034 μeV ± 0.162 μeV, with systematic uncertainties dominated by the uncertainties of the reference energies of the Li-like and Be-like uranium transitions. The experimental results are in good agreement with recent multi-configuration Dirac-Fock (MCDF) calculations and ab initio QED calculations but disagree with older predictions based on relativistic configuration interaction (RCI) methods and unified approaches. The method of double reference (moving and stationary) used in the experiment allows for control and reduction of systematic uncertainties related to the relativistic velocity of the stored ions.This article presents an experiment designed to test higher-order quantum electrodynamics (QED) effects and electron-electron interactions in highly charged uranium ions. The experiment uses a multi-reference method based on Doppler-tuned X-ray emission from stored relativistic uranium ions with different charge states. The energy of the 1s1/22p3/2J = 2 → 1s1/22s1/2J = 1 intrashell transition in the heaviest two-electron ion (U90+) is measured with an accuracy of 37 ppm. By comparing uranium ions with different numbers of bound electrons, the experiment disentangles and tests separately one-electron higher-order QED effects and bound electron-electron interaction terms without the uncertainty related to the nuclear radius. The experimental results provide a significant benchmark for calculations in strong electromagnetic fields and can discriminate between various theoretical approaches. The experiment is performed at the Experimental Storage Ring (ESR) at GSI in Darmstadt, where a beam of H-like uranium ions is stored, cooled, and decelerated to an energy of 41.03 MeV/u. The X-ray transitions are detected using two high-resolution crystal spectrometers placed at observation angles of θ = ±90° near the gas-target chamber. The energy differences between intrashell transitions in He-like, Li-like, and Be-like uranium ions are obtained, allowing for the separation of one-electron and many-electron QED effects. The final value of the He-like U transition energy is EHe = 4.509,763 ± 0.034 μeV ± 0.162 μeV, with systematic uncertainties dominated by the uncertainties of the reference energies of the Li-like and Be-like uranium transitions. The experimental results are in good agreement with recent multi-configuration Dirac-Fock (MCDF) calculations and ab initio QED calculations but disagree with older predictions based on relativistic configuration interaction (RCI) methods and unified approaches. The method of double reference (moving and stationary) used in the experiment allows for control and reduction of systematic uncertainties related to the relativistic velocity of the stored ions.