While starting up AEM instance Getting 503 ERROR

[#1]

HTTP ERROR: 503

Problem accessing /. Reason:

    AuthenticationSupport service missing. Cannot authenticate request.

[#2 · Accepted solution]

rajs73651622 This is a very generic message and definitely not enough to point out the problem. However, this may signify some sort of repository inconsistency.

I would suggest you to check for SegmentNotFound errors in the error.log file and in case you find those you can refer to [1] in order to resolve the segment store inconsistencies.

[1] Fix Inconsistencies in the repository when SegmentNotFound Issue is reported in AEM 6.x