[Help] Persistent MDM Lock on iMac Despite Release – Apple and Former Owner Cannot Remove It

Hey everyone,

I'm dealing with a frustrating issue and hope someone here has a solution or has dealt with something similar.

I bought a 21.5” Late 2015 iMac (iMac16,2, Core i5, 16 GB RAM) in 2023 via Troostwijk Auctions and installed macOS 15.xx using OpenCore Legacy Patcher. Everything went fine until setup, where I received an MDM message saying the device is managed by Windesheim University (Netherlands).

The situation:

• I contacted Windesheim, and they confirmed the iMac was fully released from Apple School Manager (ASM) on March 21, 2023.
• They provided a screenshot showing the iMac was removed from their organization.
• Apple Support tells me only Windesheim can remove the MDM lock, despite them confirming they no longer manage the device.
• Windesheim also claims they can’t do anything further — the iMac is no longer in their system.

So now I’m stuck: - Apple can’t help. - Windesheim can’t help. - The iMac is still MDM-locked and unusable.

My question:

What are my options here? Has anyone Success fully resolved a situation where a Mac remained MDM-locked after being officially released from ASM or ABM?

Any help or insights would be appreciated — I legally own the device and just want to use it.

Specs: - iMac16,2 – Late 2015 – 2.8 GHz Quad-Core i5 – 16 GB RAM
- macOS 15.xx via OpenCore Legacy Patcher
- MDM status: locked, former owner confirmed release

Thanks in advance!