I've also been having this problem and this is not a solution merely a work around that I've had success with. Click Mobile Devices in the JSS and locate the device you're having the issue thats assigned to this particular user. Select it and go to User & Location settings. Click edit and remove the user data associated to the device, user name, full name, email address. I leave the department and building as is because I don't wanna muck things up completely then click save. Go back and click Users at the top and search for the client that supposedly had no device assigned. They should indicate 0 devices assigned to them now, select and delete the user. Now again go back to Mobile Devices up top and find the iPad you removed the client data from and repopulate the fields with the client data. We use LDAP data in our client search so I just need the user name, click save when done. Check Users again and the client should have returned with a device assigned to them. Try sending out the invitation again and see if it works this time.
I've had success with this but a few times I've had to do it twice for the same device. Like it said its a work around but clearly "Houston we have a problem".
