BuddyPress.org

We figured out the problem. There was a bad SQL query.

The file we had to edit : …wp-contentpluginsbuddypressbp-activitybp-activity-classes.php

The offending code was on line 359:
$fullname_where = “AND pd.user_id = a.user_id AND pd.field_id = 1

The trick was to change the line to the following:
$fullname_where = “AND pd.user_id = a.user_id

The short explanation is that no records exists in the wp_bp_xprofile_data table that have a field_id value of ‘1’.

We don’t know how this happened – during setup? We also aren’t yet sure if this change will cause adverse effects somewhere else, but replies are now working.

Hope this helps someone else.