What Wildbook are you working in? ACW
What is the entire URL out of the browser, exactly where the error occurred?
In the metadata section of any encounter record, there’s an option to assign the encounter to a different user ID. This list still has all of the “Anonymous_” IDs that @MarkF has hidden for us elsewhere in ACW (per this older ticket: Search by submitter disappeared & replaced by oddly populating "assigned to" option - #16 by MarkF)
Would it be possible to apply the same fix from that earlier ticket to this list in the encounter/metadata section?