It seems the way Muster was done on UI was confusing to some players, even though it was working fine, so I had to change it again.
After next restart:
- Another change to Muster. Now to USE Muster, you have to click the card shadow showing above the panel used to drive game flow (where you press "Pass" and accept your choices).
So, for example in case of Muster Frodo, first you will have to click on Frodo to exert it, then on the shadow of Frodo (or other card with Muster that will show up above the panel) to actually do the Muster action.