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I have several devices that have previously been enrolled in Intune as Hybrid Azure AD Joined after which the licenses for Intune expired. Several months later, the Intune licenses are active again, but some devices seem to experience issues syncing.
I checked the Event Log and this error corresponds to MDM Session: OMA-DM message failed to be sent. Result: (Bad request (400).).
However Error 400 is quite generic, what can I do to troubleshoot this?
I understand, thanks. Here's the thread in Technet where the discussion will probably continue.
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im a bit confused -
in the task manager - under EnterpriseMgmt - there is a folder with a string of numbers( ID? ) there are multiple task in that folder - delete all of them ?
In the registry editor - under LOCAL MACHINE\SOFTWARE\ENROLLMENTS - among lots of numbered folders- found the on with the same string of number from the task manager - there are multiple folders in it- but for a key(among many) named UPN with my e-mail address - delete that key ?
Yes, you should delete the key in Registry Editor that matches the ID in EnterpriseMgmt. I assume that there will be only one match between the two. All the scheduled tasks within that group must be deleted.
delete the folder ? or the tasks that are in that folder ?
that specific key - UPN ? or the intire folder ? 
Here's the solution, step by step:
1. Open Scheduled Tasks on the affected machine
a. Microsoft - Windows - EnterpriseMgmt
b. Find the ID with the scheduled tasks
c. Delete all the scheduled tasks
2. Open Registry Editor
a. HKLM - Software - Microsoft - Enrollments
b. Find the ID from 1b) and make sure the UPN REG_SZ value under it is fooUser@tenant.onmicrosoft.com
c. Delete the key
3. Refresh Group Policy for the computer settings
a. Invoke-GPUpdate -Computer COMPUTERNAME -RandomDelayInMinutes 0
4. Make sure the Intune device enrollment is successful by checking the device in the Intune portal
5. Check Sync from Settings - Access work or school - Info
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I was having the exact same issue with just one user account. Even if I signed into different desktops, with that account, still no sync.
Turned out it was the MS License assigned to that account that was the issue.
They had been assigned an Office 365 E3 license. Once I upped them to an E5 license. Sync was successful on all desktops that earlier has sync failures.
![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(176, 59%, 27%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3ENQ%3C/text%3E%3C/svg%3E)
Thanks for your information, it's solved my problem !
May also want to ask for help in dedicated Intune forum here.
https://social.technet.microsoft.com/Forums/en-US/home?forum=microsoftintuneprod
--please don't forget to Accept as answer if the reply is helpful--
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
Disclaimer: This posting is provided "AS IS" with no warranties or guarantees, and confers no rights.
I'll do, thanks! I thought the Technet forums are being deprecated in favor of Q&A.
Sounds good, you're welcome. They will be but the process is staged to happen slowly over the next few months.