Using the '@ mention' in a Task Conversation causes the conversation window to close (converts back to the main window) without allowing the user to post it
STEPS TO REPRODUCE:
You should be able to select a Resource, and then enter Conversation text
The Conversation Window Closes
Release : 15.8, 15.8.1, 15.9
Component : CA PPM APPLICATION
This is associated to Defect DE58567 which is slated for resolution in the upcoming release, 15.9.1.
Workaround: Avoid using the @ to notify others.