Is it possible that some of the descaling compound is stuck in the boiler or dispersion disc/showerscreen area?
Try running some water though the steam wand into a cup and some water through the group into another cup and see if either tastes sour.
If its only sour through the group, then try taking off the showerscreen and dispersion disc and see if water through the bare group still tastes sour - if not, give the showerscreen and dispersion disc a thorough scrub.
If its sour through the steam wand as well as the group , then the boiler needs a more thorough rinse out.
If neither the water from the steam wand nor the group are sour, then something is going on with the grind/beans.
This is all assuming that the brew temp and shot time is fine.