events are logged for SEPM as well as for SEP in the event viewer
Symantec Endpoint Protection Manager 12.1.x event log entries
https://support.symantec.com/en_US/article.TECH196455.html
SEP Events
https://support.symantec.com/en_US/article.TECH186925.html
as support mentioned its not possible to find the root cause now as the logs would have been overwritten.
In future when it fails to import the defs into DB, you should check these two logs
Log.liveupdate and SESMLu.log
log.liveupdate is to see if SEPM was able to download, SESMLu.log to see if it was able process.
If your DB is on SQL then most of the times it would fail due to space issue. set SEPM DB to unrestricted growth
If you are running old version of SEPM, please upgrade to 12.1.6 MP4.