dfk41 - he only thing holding the screen there is a rubber gasket, similar to an E61, but a lot tighter.
It will happen if you don’t remove the shower screen often.
Best way to do this:
- remove the lever handle and he pins. Put it aside.
- remove the group from the body - two bolts, support the group and be careful when removing not to bend the pipe.
- put the group into a soft surface, and push the piston rod down. You might need to use a rubber mallet or use a piece of wood to push it down.
When popping the gasket back, lubricate the top of the gasket and the sides with food safe silicone grease. DO NOT side facing the portafilter, otherwise you’ll have a portafilter sneeze.
Do not overtighten the group bolts.
I’ve done this many times. It’s a lot simpler than it seems.