I too had a custom user field named "Source".
I upgraded to v76 per this thread and now my custom field is back, but all of the "Acceptable Values" are gone from the "Manage Library Fields Screen".
They still show in the drop-down list, but not in the "Manage Library Fields Screen", therefore, I can't modify or add any new values.
I can manually recreate them, but I was hoping there is an easier way. They must still exist somewhere in MC, because they show up in the drop-down list.
TIA