Alright, for anyone experiencing the same error as I described above; I managed to fix it!

I succeeded simply by disabling the Patchmanager and Gallery Zoom patch repositories (Sfiet_Konstantin and Ancelad, respectively). After I did this, I did a 'pkcon refresh' in Terminal, and magically Patchmanager started without a problem. I then was able to remove the rebelling Gallery Zoom patch by performing 'pkcon remove sailfishos-gallery-zoom'.

All works fine now. Hope this can be of use to anybody else