Not being a Miss Silvia user I can offer nothing other than - Your sure the batch of Beans are really OK ? But then again it would be strange for the beans to go off that bad overnight..
The longer start to pour hints at Tamp / FINE grind as you yourself suggested..
So I guess I have not really contributed to a solution.. As your TS steps are exactly what I would have been trying..
With any luck a CS with more experience on the Miss Silvia may get back from Hols soon.