Why does Assigned To: for an Activity to any admin, and only one other user not work since 4.6.7?
In the any thoughts category: If you can't find it in 4.6.8 demo and you mention a rollback. A common problem with rollbacks is that when a table is created during an upgrade and you use a db dump to roll back to previous version, the created table stays in the database. If you then redo the upgrade, it does not expect that table to be there. So it could leave you with a system that is in disbalance. 

So, my question is, when you did the rollback, did you remove and recreate the DB or did you just apply the db dump (with the risk of leaving a created table behind)?

If you did the latter: Do you still have your 4.6.6 DB? If so, could you immediately upgrade it to 4.6.8 and see if the problem still exists? If it doesn't, you could compare the table structures of your current live DB and this upgraded DB.