Hi,I resolved this issue by setting up NTP on our core switch 6513 and pointing ISE towards that.  We got the same errors you are seeing when using an AD server for NTP.  Since moving I have not gotten any NTP errors.

I am also having this issue.  Every 15 minutes I get alarms.  Tried differnt NTp servers 0 AD, external, etc.. with no luck.  I am hoping someone has made progress in resolving this issue.