Error 500 when importing contacts from csv file

When trying to import additional contacts from a csv file, I have error page 500.

There is the following error in the logs:

mautic.CRITICAL: Uncaught PHP Exception Symfony\Component\Debug\Exception\FatalThrowableError: “strpos() expects parameter 1 to be string, int given” at /home/user/public_html/mautic/app/bundles/LeadBundle/EventListener/ImportContactSubscriber.php line 199 {“exception”:“[object] (Symfony\Component\Debug\Exception\FatalThrowableError(code: 0): strpos() expects parameter 1 to be string, int given at /home/user/public_html/mautic/app/bundles/LeadBundle/EventListener/ImportContactSubscriber.php:199)”} {“hostname”:“l2.host.pl”,“pid”:8660}