Which radius server are you using? NPS, Cisco ISE, Freeradius?
What I can suggest is to keep your server always updated, and review which services are stopping during the problem.
To be honest, it doesn't seem to have anything to do with Meraki. So it would be good to talk to a server expert.
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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