Ithenticate plugin 1.0.6.1 breaks article submission in OJS 3.3.0-17

We run OJS 3.3.0-17 and want to use the Ithenticate plugin in version 1.0.6.1 from the plugin gallery. We have a paid Ithenticate account (with enough remaining articles) and are using the correct credentials (Ithenticate login via Ithenticate website is possible). We tested this with both a regular user and an admin user, both have all the necessary rights (i.e. uploads, folders, folder groups).

When we setup the plugin in the OJS backend, submission is no longer possible at step 4, we just get an endless spinner (i.e. authors cannot finish submission!). When we setup the plugin in config.inc.php, the submission goes through but nothing arrives in the Ithenticate account. Using the backend, no error message is created, but using config.inc, we get the following error message:

[Wed Sep 18 05:12:17.291583 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/themes/default/DefaultThemePlugin.inc.php on line 163
[Wed Sep 18 05:12:23.322969 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/themes/default/DefaultThemePlugin.inc.php on line 163
[Wed Sep 18 05:12:24.191216 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/themes/default/DefaultThemePlugin.inc.php on line 163, referer: https://OURDOMAIN/index.php/sig/issue/current
[Wed Sep 18 05:12:25.990784 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/themes/default/DefaultThemePlugin.inc.php on line 163
[Wed Sep 18 05:12:32.668782 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/themes/default/DefaultThemePlugin.inc.php on line 163
[Wed Sep 18 10:19:08.008769 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type int in /var/www/OURFOLDER/html/plugins/generic/plagiarism/vendor/bsobbe/ithenticate/Ithenticate.php on line 135, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099
[Wed Sep 18 10:19:08.008803 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/generic/plagiarism/vendor/bsobbe/ithenticate/Ithenticate.php on line 135, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099
[Wed Sep 18 10:19:08.008806 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/generic/plagiarism/vendor/bsobbe/ithenticate/Ithenticate.php on line 135, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099
[Wed Sep 18 10:19:08.008808 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/generic/plagiarism/vendor/bsobbe/ithenticate/Ithenticate.php on line 135, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099
[Wed Sep 18 10:19:08.008811 2024] [php:warn] [pid 3722754] [client IP:PORT] PHP Warning:  Trying to access array offset on value of type null in /var/www/OURFOLDER/html/plugins/generic/plagiarism/vendor/bsobbe/ithenticate/Ithenticate.php on line 135, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099
[Wed Sep 18 10:19:08.008857 2024] [php:error] [pid 3722754] [client IP:PORT] PHP Fatal error:  Uncaught TypeError: array_search(): Argument #2 ($haystack) must be of type array, bool given in /var/www/OURFOLDER/html/plugins/generic/plagiarism/PlagiarismPlugin.inc.php:132\nStack trace:\n#0 /var/www/OURFOLDER/html/plugins/generic/plagiarism/PlagiarismPlugin.inc.php(132): array_search()\n#1 /var/www/OURFOLDER/html/lib/pkp/classes/plugins/HookRegistry.inc.php(107): PlagiarismPlugin->callback()\n#2 /var/www/OURFOLDER/html/lib/pkp/classes/form/Form.inc.php(320): HookRegistry::call()\n#3 /var/www/OURFOLDER/html/lib/pkp/classes/submission/form/PKPSubmissionSubmitStep4Form.inc.php(45): Form->execute()\n#4 /var/www/OURFOLDER/html/classes/submission/form/SubmissionSubmitStep4Form.inc.php(36): PKPSubmissionSubmitStep4Form->execute()\n#5 /var/www/OURFOLDER/html/lib/pkp/pages/submission/PKPSubmissionHandler.inc.php(177): SubmissionSubmitStep4Form->execute()\n#6 /var/www/OURFOLDER/html/lib/pkp/classes/core/PKPRouter.inc.php(397): PKPSubmissionHandler->saveStep()\n#7 /var/www/OURFOLDER/html/lib/pkp/classes/core/PKPPageRouter.inc.php(246): PKPRouter->_authorizeInitializeAndCallRequest()\n#8 /var/www/OURFOLDER/html/lib/pkp/classes/core/Dispatcher.inc.php(144): PKPPageRouter->route()\n#9 /var/www/OURFOLDER/html/lib/pkp/classes/core/PKPApplication.inc.php(360): Dispatcher->dispatch()\n#10 /var/www/OURFOLDER/html/index.php(68): PKPApplication->execute()\n#11 {main}\n  thrown in /var/www/OURFOLDER/html/plugins/generic/plagiarism/PlagiarismPlugin.inc.php on line 132, referer: https://OURDOMAIN/index.php/wks/submission/wizard/2?submissionId=4099

This seems to suggest that there is a problem with folder group creation, but it does not have to do with incorrect credentials. We looked through the other Ithenticate forum threads but apparently our situation has not yet been encountered or posted. Any help would be appreciated.