In Cisco DNA center, version 2.3.3.6, under System -> Settings -> Trust & Privacy -> Device Certificate and PKI Certificates, you get an error message saying:

Internal Server Error: An unexpected condition was encountered. Please try after the system is restored.

Current solution is to restart pki broker and jboss service from DNAC shell.

magctl service restart -d apic-em-pki-broker-service
magctl service restart -d apic-em-jboss-ejbca

Wait a few minutes after restart and try again.

This is probably related to this bug: https://bst.cisco.com/bugsearch/bug/CSCwd25799