UPDATE: the error log was showing the following error:

[16-Mar-2021 11:43:13 UTC] PHP Parse error: syntax error, unexpected '->' (T_OBJECT_OPERATOR) in [path_to_una]modules/boonex/events/classes/BxEventsDb.php on line 146

As Stanislav Mitov had the 5.6 version of PHP in his server, he was advised to switch it to a more new variant. With PHP 7.3 it works fine now.