Could be the angle that the chain is making as it feeds onto the lower pulley. If this is the case then this should be much more of an issue when the big chainring is in use and the chain is crossing more sharply.

The big cog is also suspect here as was mentioned.

The Allvit has no problem with your gearing, but will shift faster/crisper if a bigger freewheel is used.

One can slightly adjust the body angle and improve shifting response by grinding away at the stop on the claw bracket (I've used a thin round file for this), but remove too much metal there and the protrusion may break off.
Remember that old derailers were designed with stiff, bushed chains in mind, so modern chain does not transmit cage motion into pushing the chain around very effectively unless the chain gap is kept small. The Allvit works best with today's chains over a 14-28t, 5s freewheel.

The cable housing length looks to be exactly as I would cut it, you want the loop to be minimal for the cable to have a smoothest path and for the housing to flex as little as possible as the lever is moved!