If the "Prompt for comment on checkout" is disabled. It no more possible to set an activity. On checkouts the set activity will be chosen by default. But If there's no activity set there is no UI feedback.
When I launch the plugin in debugging mode I can see the cleartool errors :
launching: cleartool co -nwa -nc -pti -res D:\Views\SCM_02\tmp.txt>>> co -nwa -nc -pti -res D:\Views\SCM_02\tmp.txt
<<# cleartool: Error: To operate on UCM branch, must be set to an activity and a UCM view.
<<# cleartool: Error: Unable to check out "D:\Views\SCM_02\tmp.txt".
It could be nice to open a dialog window to enable choosing an activity when no activity is set.
We can also imagine a preference tab "Activity" with "Prompt for an activity on checkout/in/add/auto-checkout".