You can try a few things, such as verifying the ADE token in ABM and Meraki, creating a new ADE profile with authentication disabled, restoring the iPad to factory settings and retrying setup, and avoiding URL enrollment unless you're using manual post-setup enrollment.
If the issue persists, I suggest you open a Meraki support case.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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