The following or similar error might be throwing when trying to Undo Deprovisioning an user.
"Administrative Policy returned an error. 'Built-in Policy - Azure - Default Rules to Generate Properties' for 'edsaAzureUserUPNPrefix' returned an error Object reference not set to an instance of an object."
The user was Deprovisioned with the default "Built-in Policy - User Default Deprovisioning" policy blocked.
The Change History will show as follow: "The object is changed to the "Deprovisioned" state. No other changes are made to the object. Reason: No deprovisioning policies are in effect on this object."
Clear up the edsvaDeprovisionStatus attribute from the failing user and enable the account.