Not working on NC 21

The current version does appear in the menu bar, but always returns nothing found. When my cronjob gets executed it throws a wrning:
Private methods cannot be final as they are never overridden by other classes in /path/to/cloud/lib/private/FullTextSearch/Model/IndexDocument.php, line 725.

Also Elasticsearch returns an error repeatedly:

Errors

```output {"reqId":"YEnOlw6jH8HBeU8UagTt6wAAAJE","level":3,"time":"2021-03-11T08:02:31+00:00","remoteAddr":"2003:df:4f30:400:4d9f:b389:f806:d58a","user":"TheColin21","app":"PHP","method":"GET","url":"/index.php/apps/fulltextsearch/v1/search?request=%7B%22providers%22%3A%5B%22files%22%5D%2C%22options%22%3A%7B%22files_local%22%3A%220%22%2C%22files_extension%22%3A%22%22%7D%2C%22search%22%3A%22willy%22%2C%22page%22%3A1%7D","message":{"Exception":"Error","Message":"Elasticsearch\\Connections\\Connection::Elasticsearch\\Connections\\{closure}(): Argument #2 ($key) must be passed by reference, value given at /path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Connections/Connection.php#358","Code":0,"Trace":[{"function":"onError","class":"OC\\Log\\ErrorHandler","type":"::"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Connections/Connection.php","line":358,"function":"array_walk"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Connections/Connection.php","line":218,"function":"getURI","class":"Elasticsearch\\Connections\\Connection","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Transport.php","line":110,"function":"performRequest","class":"Elasticsearch\\Connections\\Connection","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Client.php","line":1637,"function":"performRequest","class":"Elasticsearch\\Transport","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/vendor/elasticsearch/elasticsearch/src/Elasticsearch/Client.php","line":1220,"function":"performRequest","class":"Elasticsearch\\Client","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/lib/Service/SearchService.php","line":100,"function":"search","class":"Elasticsearch\\Client","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch_elasticsearch/lib/Platform/ElasticSearchPlatform.php","line":336,"function":"searchRequest","class":"OCA\\FullTextSearch_Elasticsearch\\Service\\SearchService","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch/lib/Service/SearchService.php","line":197,"function":"searchRequest","class":"OCA\\FullTextSearch_Elasticsearch\\Platform\\ElasticSearchPlatform","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch/lib/Service/SearchService.php","line":161,"function":"searchFromProviders","class":"OCA\\FullTextSearch\\Service\\SearchService","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch/lib/Controller/ApiController.php","line":120,"function":"search","class":"OCA\\FullTextSearch\\Service\\SearchService","type":"->"},{"file":"/path/to/cloud/apps/fulltextsearch/lib/Controller/ApiController.php","line":95,"function":"searchDocuments","class":"OCA\\FullTextSearch\\Controller\\ApiController","type":"->"},{"file":"/path/to/cloud/lib/private/AppFramework/Http/Dispatcher.php","line":218,"function":"search","class":"OCA\\FullTextSearch\\Controller\\ApiController","type":"->"},{"file":"/path/to/cloud/lib/private/AppFramework/Http/Dispatcher.php","line":127,"function":"executeController","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/AppFramework/App.php","line":157,"function":"dispatch","class":"OC\\AppFramework\\Http\\Dispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/Route/Router.php","line":302,"function":"main","class":"OC\\AppFramework\\App","type":"::"},{"file":"/path/to/cloud/lib/base.php","line":993,"function":"match","class":"OC\\Route\\Router","type":"->"},{"file":"/path/to/cloud/index.php","line":37,"function":"handleRequest","class":"OC","type":"::"}],"File":"/path/to/cloud/lib/private/Log/ErrorHandler.php","Line":92,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.72 Safari/537.36 Edg/89.0.774.45","version":"21.0.0.18","id":"6049cf2c52922"}

FullTextSearch also repeatedly throws this error:
{"reqId":"YEnN3VaaWpYN6lnwgvCCGAAAAEo","level":3,"time":"2021-03-11T07:59:25+00:00","remoteAddr":"2003:df:4f30:400:4d9f:b389:f806:d58a","user":"TheColin21","app":"no app in context","method":"DELETE","url":"/remote.php/dav/files/TheColin21/NPP_Settings/backup/index.html@2021-03-11_085831","message":{"Exception":"OCP\FullTextSearch\Exceptions\FullTextSearchAppNotAvailableException","Message":"No IIndexService registered","Code":0,"Trace":[{"file":"/path/to/cloud/lib/private/FullTextSearch/FullTextSearchManager.php","line":193,"function":"getIndexService","class":"OC\FullTextSearch\FullTextSearchManager","type":"->"},{"file":"/path/to/cloud/apps/files_fulltextsearch/lib/Listeners/FileDeleted.php","line":61,"function":"updateIndexStatus","class":"OC\FullTextSearch\FullTextSearchManager","type":"->","args":["*** sensitive parameters replaced ***"]},{"file":"/path/to/cloud/lib/private/EventDispatcher/ServiceEventListener.php","line":76,"function":"handle","class":"OCA\Files_FullTextSearch\Listeners\FileDeleted","type":"->"},{"file":"/path/to/cloud/3rdparty/symfony/event-dispatcher/EventDispatcher.php","line":251,"function":"__invoke","class":"OC\EventDispatcher\ServiceEventListener","type":"->"},{"file":"/path/to/cloud/3rdparty/symfony/event-dispatcher/EventDispatcher.php","line":73,"function":"callListeners","class":"Symfony\Component\EventDispatcher\EventDispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/EventDispatcher/EventDispatcher.php","line":86,"function":"dispatch","class":"Symfony\Component\EventDispatcher\EventDispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/EventDispatcher/EventDispatcher.php","line":98,"function":"dispatch","class":"OC\EventDispatcher\EventDispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/Files/Node/HookConnector.php","line":157,"function":"dispatchTyped","class":"OC\EventDispatcher\EventDispatcher","type":"->"},{"file":"/path/to/cloud/lib/private/legacy/OC_Hook.php","line":110,"function":"postDelete","class":"OC\Files\Node\HookConnector","type":"->"},{"file":"/path/to/cloud/lib/private/Files/View.php","line":1281,"function":"emit","class":"OC_Hook","type":"::"},{"file":"/path/to/cloud/lib/private/Files/View.php","line":1209,"function":"runHooks","class":"OC\Files\View","type":"->"},{"file":"/path/to/cloud/lib/private/Files/View.php","line":725,"function":"basicOperation","class":"OC\Files\View","type":"->"},{"file":"/path/to/cloud/apps/dav/lib/Connector/Sabre/File.php","line":466,"function":"unlink","class":"OC\Files\View","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/dav/lib/DAV/Tree.php","line":179,"function":"delete","class":"OCA\DAV\Connector\Sabre\File","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/dav/lib/DAV/CorePlugin.php","line":281,"function":"delete","class":"Sabre\DAV\Tree","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/event/lib/WildcardEmitterTrait.php","line":89,"function":"httpDelete","class":"Sabre\DAV\CorePlugin","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/dav/lib/DAV/Server.php","line":472,"function":"emit","class":"Sabre\DAV\Server","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/dav/lib/DAV/Server.php","line":253,"function":"invokeMethod","class":"Sabre\DAV\Server","type":"->"},{"file":"/path/to/cloud/3rdparty/sabre/dav/lib/DAV/Server.php","line":321,"function":"start","class":"Sabre\DAV\Server","type":"->"},{"file":"/path/to/cloud/apps/dav/lib/Server.php","line":332,"function":"exec","class":"Sabre\DAV\Server","type":"->"},{"file":"/path/to/cloud/apps/dav/appinfo/v2/remote.php","line":35,"function":"exec","class":"OCA\DAV\Server","type":"->"},{"file":"/path/to/cloud/remote.php","line":167,"args":["/path/to/cloud/apps/dav/appinfo/v2/remote.php"],"function":"require_once"}],"File":"/path/to/cloud/lib/private/FullTextSearch/FullTextSearchManager.php","Line":117,"CustomMessage":"--"},"userAgent":"Mozilla/5.0 (Windows) mirall/3.1.3stable-Win64 (build 20210218) (Nextcloud)","version":"21.0.0.18","id":"6049cf2c55333"}

```

Nextcloud 21.0.0 on Ubuntu 20.04 with PHP 8.0

Description

Contributor guide

First steps

Research direction

Assessment