I can reproduce this when assigning a ticket to a moderator who has no permission to view the ticket.
The current reassign view assumes that you're going to assign to a moderator / admin but that's a bit silly considering there are different use cases, etc.
There are two options here, either change the dropdown list to a freeform textbox that accepts usernames. On submission of the form, the system will check if the user has permission to be assigned to that ticket and assign it to them. Alternatively, allow anyone to be assigned to a ticket and grant them temporary view and reply privileges while they are assigned.
Thoughts?