We have developed an advanced ultraviolet (UV) spectropolarimeter called Chromospheric Layer SpectroPolarimeter (CLASP2), designed to explore the magnetic fields in the upper solar chromosphere. CLASP2 was launched on board a NASA sounding rocket experiment in 2019, and successfully detected the full Stokes vector in an active-region (AR) plage and in the quiet Sun near the limb across the Mg II h and k lines (280 nm) for the first time. After the experiment, CLASP2 was fully recovered and conducted a second experiment in 2021 under the new name CLASP2.1. During this experiment, we scanned the AR NOAA 12882 with a field of view of 28 arcseconds by 196 arcseconds. By analyzing the CLASP2.1 flight data, we have detected for the first time a magnetic discontinuity in the upper solar chromosphere associated with a coronal loop brightening. During 6-minutes flight of CLASP2.1, we find a coronal loop brightening that is intermittently and recurrently heated. The temperature of the brightening plasma is above 2.5 MK, which is higher than the ambient temperature, and no corresponding brightening is detected in the images of the photosphere and lower chromosphere. The notable finding of our study is that the coronal loop brightening of interest is observed along a region where the magnetic field polarity changes abruptly in the upper chromosphere. Our findings provide observational evidence that the heating mechanism of the coronal loop brightening is consistent with magnetic reconnection in the upper solar chromosphere.