FTS check fails: "TypeError: array_shift() expects parameter 1 to be array, string given" after upgrade

Hello!
After Nextcloud upgrade, I am having issues running indexing as well as the basic test script.

Here's the output:

.Testing your current setup:  
Creating mocked content provider. ok  
Testing mocked provider: get indexable documents. (2 items) ok  
Loading search platform. (Elasticsearch) ok  
Testing search platform. ok  
Locking process ok  
Removing test. ok  
Pausing 3 seconds 1 2 3 ok  
Initializing index mapping. ok  
Indexing generated documents. ok  
Pausing 3 seconds 1 2 3 ok  
Retreiving content from a big index (license). (size: 32386) ok  
Comparing document with source. ok  
Searching basic keywords:  
 - 'test' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple test' (result: 2, expected: ["simple","license"]) ok  
 - '"document is a test"' (result: 0, expected: []) ok  
 - '"document is a simple test"' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple -test' (result: 1, expected: ["license"]) ok  
 - 'document is a simple +test' (result: 1, expected: ["simple"]) ok  
 - '-document is a simple test' (result: 0, expected: []) ok  
 - 'document is a simple +test +testing' (result: 1, expected: ["simple"]) ok  
 - 'document is a simple +test -testing' (result: 0, expected: []) ok  
 - 'document is a +simple -test -testing' (result: 0, expected: []) ok  
 - '+document is a simple -test -testing' (result: 1, expected: ["license"]) ok  
 - 'document is a +simple -license +testing' (result: 1, expected: ["simple"]) ok  
Updating documents access. An unhandled exception has been thrown:
TypeError: array_shift() expects parameter 1 to be array, string given in /var/www/html/cloud/apps/fulltextsearch/lib/Service/IndexService.php:297
Stack trace:
#0 /var/www/html/cloud/apps/fulltextsearch/lib/Service/IndexService.php(297): array_shift('*** sensitive p...')
#1 /var/www/html/cloud/apps/fulltextsearch/lib/Service/IndexService.php(204): OCA\FullTextSearch\Service\IndexService->indexDocuments(Object(OCA\FullTextSearch_ElasticSearch\Platform\ElasticSearchPlatform), Object(OCA\FullTextSearch\Provider\TestProvider), '*** sensitive p...', Object(OCA\FullTextSearch\Model\IndexOptions))
#2 /var/www/html/cloud/apps/fulltextsearch/lib/Command/Test.php(506): OCA\FullTextSearch\Service\IndexService->indexProviderContentFromUser(Object(OCA\FullTextSearch_ElasticSearch\Platform\ElasticSearchPlatform), Object(OCA\FullTextSearch\Provider\TestProvider), 'user1', Object(OCA\FullTextSearch\Model\IndexOptions))
#3 /var/www/html/cloud/apps/fulltextsearch/lib/Command/Test.php(172): OCA\FullTextSearch\Command\Test->testUpdatingDocumentsAccess(Object(Symfony\Component\Console\Output\ConsoleOutput), Object(OCA\FullTextSearch_ElasticSearch\Platform\ElasticSearchPlatform), Object(OCA\FullTextSearch\Provider\TestProvider))
#4 /var/www/html/cloud/3rdparty/symfony/console/Command/Command.php(255): OCA\FullTextSearch\Command\Test->execute(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#5 /var/www/html/cloud/core/Command/Base.php(169): Symfony\Component\Console\Command\Command->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#6 /var/www/html/cloud/3rdparty/symfony/console/Application.php(1000): OC\Core\Command\Base->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#7 /var/www/html/cloud/3rdparty/symfony/console/Application.php(271): Symfony\Component\Console\Application->doRunCommand(Object(OCA\FullTextSearch\Command\Test), Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#8 /var/www/html/cloud/3rdparty/symfony/console/Application.php(147): Symfony\Component\Console\Application->doRun(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#9 /var/www/html/cloud/lib/private/Console/Application.php(215): Symfony\Component\Console\Application->run(Object(Symfony\Component\Console\Input\ArgvInput), Object(Symfony\Component\Console\Output\ConsoleOutput))
#10 /var/www/html/cloud/console.php(100): OC\Console\Application->run()
#11 /var/www/html/cloud/occ(11): require_once('/var/www/html/c...')

This is my config:

Full text search 2.0.0
 
- Search Platform:
Elasticsearch 2.0.0
{
    "elastic_host": [
        "http://127.0.0.1:9200"
    ],
    "elastic_index": "nc",
    "fields_limit": "10000",
    "es_ver_below66": "0",
    "analyzer_tokenizer": "standard"
} 
 
- Content Providers:
Files 2.0.0
{
    "files_local": "1",
    "files_external": "0",
    "files_group_folders": "1",
    "files_encrypted": "0",
    "files_federated": "0",
    "files_size": "100",
    "files_pdf": "1",
    "files_office": "1",
    "files_image": "0",
    "files_audio": "0"
}

I'm using elasticsearch Version: 7.10.0, and Nextcloud 20.0.2 - what could be the problem?

Start at lib/Service/IndexService.php:297 and follow the calls from lib/Command/Test.php shown in the stack trace. Reproduce the failure with the basic test/indexing flow against Elasticsearch 7.10.0, then inspect the value passed to array_shift during access updates. Done means the test completes without the TypeError and indexing remains functional.