Closing for now as there is no repository corruption and the error is gone now without the need to restart gerrit.

Thanks for reporting this, indeed it looks like the same "missing unknown" issue that seems to be caused by minor differences in CGit and JGit garbage collection implementations:

[2018-10-14 00:07:08,678] [HTTP-1226] ERROR com.google.gerrit.httpd.restapi.RestApiServlet : Error in GET /changes/91400/revisions/05c5fd44380bd28b36670c75c61aa878d4e79621/commit?links
org.eclipse.jgit.errors.MissingObjectException: Missing unknown 05c5fd44380bd28b36670c75c61aa878d4e79621
at org.eclipse.jgit.internal.storage.file.WindowCursor.open(WindowCursor.java:163)
at org.eclipse.jgit.lib.ObjectReader.open(ObjectReader.java:234)
at org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:859)
at org.eclipse.jgit.revwalk.RevWalk.parseCommit(RevWalk.java:772)
at com.google.gerrit.server.change.GetCommit.apply(GetCommit.java:51)

I am planning to move Gerrit to EL7 which will get us to a newer system git version and hopefully decrease the rate of this issue happening.

I just found out the the problem disappeared, and I was able to merge the change.
However, please look into gerrit log - there might be a worse issue hiding.