I have a reporter field in ConfiForms set to User field type as follows:

For SOME users (working for me when I try to submit the form) the following error occurs indicating that the reporter field cannot be set

IFTTT Rule
#set($typeid = "[entry.type.id]")
{
"fields": {
"project": { "key": "XX" },
"issuetype": { "name": "Story" },
"priority": { "name": "[entry.priority.label]" },
"reporter": { "name": "[entry.reporter.username]" },
...
}
}