  We report on a remarkable change in the spectral energy distribution (SED) of
Mrk 841, providing new insights on how the soft X-ray excess emission in active
galactic nuclei (AGNs) is produced. By Swift monitoring of a sample of
Seyfert-1 galaxies, we found an X-ray spectral hardening event in Mrk 841. We
thereby triggered our XMM-Newton, NuSTAR, and HST observations in 2022 to study
this event. Our previous investigations of such events in other AGNs had shown
that they are caused by obscuring winds. However, the event in Mrk 841 has
different spectral characteristics and origin. We find it is the soft X-ray
excess component that has become dimmer. This is, importantly, accompanied by a
similar decline in the optical/UV continuum, suggesting a connection to the
soft X-ray excess. In contrast, there is relatively little change in the X-ray
power-law and the reflection components. Our SED modeling suggests that the
soft X-ray excess is the high-energy extension of the optical/UV disk emission,
produced by warm Comptonization. We find the temperature of the disk dropped in
2022, explaining the observed SED dimming. We then examined the Swift data,
taken over 15 years, to further decipher the UV and X-ray variabilities of Mrk
841. A significant relation between the variabilities of the X-ray spectral
hardness and that of the UV continuum is found, again suggesting that the soft
excess and the disk emission are interlinked. This is readily explicable if the
soft excess is produced by warm Comptonization.
