PHP Unit failing without error message in GitLab CI/CD

Question

In GitLab we have a PHP Unit step in out CI/CD, which sometimes fails without an explanation, however when you run the same job a few minutes later it (not always) works.

Output of the job:

PHPUnit 9.5.13 by Sebastian Bergmann and contributors.
Runtime:       PHP 8.1.0
Configuration: /srv/api/phpunit.xml.dist
Warning:       No code coverage driver available
Testing 
PHP Deprecated:  Return type of Lexik\Bundle\JWTAuthenticationBundle\TokenExtractor\ChainTokenExtractor::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /srv/api/vendor/lexik/jwt-authentication-bundle/TokenExtractor/ChainTokenExtractor.php on line 91
Deprecated: Return type of Lexik\Bundle\JWTAuthenticationBundle\TokenExtractor\ChainTokenExtractor::getIterator() should either be compatible with IteratorAggregate::getIterator(): Traversable, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /srv/api/vendor/lexik/jwt-authentication-bundle/TokenExtractor/ChainTokenExtractor.php on line 91
Test 'App\Tests\Functional\GlnTest::testIsUsed' started
Authenticating with credentials from /root/.docker/config.json 00:02
ERROR: Job failed: exit code 1

The command used to run the test is: bin/phpunit --coverage-text --stop-on-failure --testsuite Functional --verbose --debug

I assume it has to have something to do with available processing power or Ram (though the server has free cores and has available ram, according to htop/free -m.

Question: What could cause this failure without any cause given? Is there a way I can debug this issue further?