Automated tests fail in latest dev version

Problem

Suddenly test fails, so some dependency must have changed.

There was 1 error:
1) Drupal\Tests\lupus_decoupled_user_form\Functional\LupusSessionDomainFunctionalTest::testLoginFormSubmission
TypeError: Drupal\Tests\lupus_decoupled_user_form\Functional\LupusSessionDomainFunctionalTest::getFormBuildId(): Argument #1 ($form_html) must be of type string, null given, called in /builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php on line 199
/builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php:260
/builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php:199
--
There were 4 failures:
1) Drupal\Tests\lupus_decoupled_user_form\Functional\LupusSessionDomainFunctionalTest::testUserLoginApi
Failed asserting that Array &0 [] is identical to 'user_login_form'.
/builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php:175
2) Drupal\Tests\lupus_decoupled_views\Functional\Views\CustomElementsViewsDisplaysTest::testCustomElementsPageView
Failed asserting that actual size 0 matches expected size 4.
/builds/project/lupus_decoupled/modules/lupus_decoupled_views/tests/src/Functional/Views/CustomElementsViewsDisplaysTest.php:103
3) Drupal\Tests\lupus_decoupled_webform\Functional\CustomElementWebformTest::testCustomElementWebform
Failed asserting that Array &0 [] is identical to 'Test: Custom elements webform'.
/builds/project/lupus_decoupled/modules/lupus_decoupled_webform/tests/src/Functional/CustomElementWebformTest.php:86
4) Drupal\Tests\lupus_decoupled\Functional\Views\CustomElementsSearchViewTest::testCustomElementsSearchPageView
Failed asserting that actual size 0 matches expected size 3.
/builds/project/lupus_decoupled/tests/src/Functional/Views/CustomElementsSearchViewTest.php:134
--
There was 1 risky test:
1) Drupal\Tests\lupus_decoupled_user_form\Functional\LupusSessionDomainFunctionalTest::testLoginFormSubmission
Test code or tested code printed unexpected output: 
Warning: Undefined array key "content" in /builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php on line 199
/builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php:195
--
1 test triggered 1 PHP warning:
1) /builds/project/lupus_decoupled/modules/lupus_decoupled_user_form/tests/src/Functional/LupusSessionDomainFunctionalTest.php:199
Undefined array key "content"
ERRORS!
Tests: 49, Assertions: 334, Errors: 1, Failures: 4, Warnings: 1, PHPUnit Deprecations: 40, Risky: 1.
$ cp /var/log/apache2/test.apache.access.log $CI_PROJECT_DIR/apache.access.log.txt
$ cp /var/log/apache2/test.apache.error.log $CI_PROJECT_DIR/apache.error.log.txt
$ exit $EXIT_CODE
Uploading artifacts for failed job

Steps to reproduce

check Gitlab CI pipelines of 1.x