Date value missing after upgrade, specially for records which were there in 'activity' table before upgrade.

Make sure dates are migrated to new date column (activity_date_time) for already existing records in activity_table, during activity upgrade process.

The reason for broken links is the hardcoded $activityTypeId in the file - CRM/Activity/Selector/Activity.php

"
elseif ( $activityTypeId == 5 ) { // event registration
$url = 'civicrm/contact/view/participant';
$qsView = "action=view&reset=1&id={$sourceRecordId}&cid=%%cid%%&context=%%cxt%%";
} elseif ( $activityTypeId == 6 ) { //contribution
$url = 'civicrm/contact/view/contribution';
$qsView = "action=view&reset=1&id={$sourceRecordId}&cid=%%cid%%&context=%%cxt%%";
} elseif ( in_array($activityTypeId, array( 7, 8 ) ) ) { // membership
$url = 'civicrm/contact/view/membership';
$qsView = "action=view&reset=1&id={$sourceRecordId}&cid=%%cid%%&context=%%cxt%%";
}
"

Check http://forum.civicrm.org/index.php?topic=3277.0 for details.