On our instance of Phabricator we have edited the maniphest.custom-field-definitions and added a field to the Maniphest task to enter a Phabricator user for the product responsibility. It looks something like:

"key:product-responsibility": {
   "name": "Product Responsibility",
   "type": "users",
   "description": "Person who is in charge for the product.",
   "copy": "true"
 }

So far this worked fine, but after an update this morning, I cannot add a user to this field anymore. It doesn't matter if I create a new task or edit an existing task, and it doesn't matter which user I enter in the field, there is always this error message:

Unhandled Exception ("InvalidArgumentException")	
Argument 1 passed to PhabricatorPolicyAwareQuery::setViewer() must be an instance of PhabricatorUser, null given, called in /opt/phabricator/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldPHIDs.php on line 166 and defined

I'm updating on the stable branch. Sadly I don't know on which version our Phabricator instance was running before, but the last update must have been about two month ago.

Any ideas? Any further information I could provide?