Advanced Search (ElasticSearch) fails when searching on issues


We have been trying to get our on-prem GitLab (latest version) to work with ElasticSearch, but have ran into an issue that we haven’t figured out how to resolve yet.

If we do a regular (basic) search for “xxx” then it matches with 7 issues (“Issues 7”).
However, if we activate Advanced Search and repeat the search then Issues have no matches.
Actually, it doesn’t say “Issues 0” but just “Issues” and if you click Issues then you get a HTPP 500 error.

gitlab-production-issues have default values for shards and replicas under Admin → General → Advanced Search.

Is there something wrong with the index for issues? Do we need to recreate it from command line?
Should we delete all indexes (again) and start all-over?

Any input would be appreciated.