I know this is late, and there is already an accepted answer, but I just had this issue and it was resolved by changing the format of the queue string.
When my queue name was this, I got the workgroup error:
".QueueName"
When I changed it to a more formal version, there was no error and sending to the queue worked:
"FormatName:DIRECT=OS:ComputerNameprivate$QueueName"
Just in case someone else comes across this post, now they have something else to try...
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