Summary: After importing the config file (exported from a GSA at older version), the indexer process fails to start, crawling stops and feeds do not get processed.
Cause: Issue #7473534 "Importing a configuration from another GSA causes crawling to stop."
The config imported was from a GSA with some multibox (Unification, GSA^n/mirroring/Distributed Crawling and Serving) related configs and GSA is not properly handling these config due to above bug, causing the indexer to fail.
Fix: On the affected GSA, you need to toggle on/off the multibox feature in order to clear up the config that causes the problem.
Go to Admin Console > GSA^n > Configuration, click on "Disable GSA^n". If the button cannot be found, then first enable mirroring, then backtrack to Configuration page to disable it.
If the issue still persists, repeat the same for the Unification by going to Admin Console > GSA Unification > "Enable GSA Unification", then "Disable GSA Unification" to toggle.
Versions affected: 6.14.0, 7.0.14, 7.2.0, 7.4.0