Ok I just have a guess. Something that will do you no harm to check...
First, do you own some cafiza? Some sort of coffee machine detergent? Its terrific stuff. You should periodically disassemble and boil group head parts in it. 20 minutes boiled in cafiza you'll be shocked how much tarry grunge is removed.
You really should be back flushing with cafiza every month or so. I'd do that first, chances are it'll clean out the clogged tar.
But if machine is really gummed up you probably should take it all apart and clean it. Much more time efficient.
Your picture shows the component that I suspect is jammed with tar. In your picture #10 is the main body of the group head. There's a bolt on top that screws in, there's another big brass tube thing that bolts onto the bottom of it, its the bottom thing that can get really gummed up. Unplug machine, let it cool, then use a wrapped wrench to undo both of these. Try and remember the parts assembly order, they go together in a very specific way. Pay particular attention to the orientation of short and long plungers, the springs, etc. They're all different.
Take all those parts out and boil in caffiza, if its really packed you might need to boil 2-3 times and scrape the tar off. Once everything is clean reassemble. I like to use some food safe lube on threads when reassembling, otherwise it can gall.
There's really no need to take machine in for this, the cleaning and assembly are really simple. Youve done 99% of the work just finding the parts diagram.