The work around for this problem is to select (LEFT-click) on any other Party Member's protrait, and then go back and select the one that wasn't responding before. This solution has worked for me whenever this happens.

The technical/coding issues seem to be all about how the game establishes which character has the "focus" of any action and what it's possible for that character to do at that specific moment. It appears that the focus gets stuck sometimes. See also the RMB ("Right Mouse Button") glitches.