I had the same issues with PanOS 8.1.10. Issue started occurring some hours after upgrade to 8.1.10, and resolved itself without measures taken after a few days. Multiple hardware types (PA-200 up to PA-5050) were involved, even when the firewall and Panorama were in the same data center. TAC mentioned that it might be related to the upgrade. In case this problem occurs again, this packet captures between firewalls and Panorama should be set up and the ms-log should be viewed. Hope this information will help progress similar cases.
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