So I am having the same issue, I was able to fix it by adding a question mark before the word string in line 120 of the code on my backend. Basically followed the menu all the way down until I got to the correct php code.
Started at the content/ and went all the way down until I found the code to correct.
An error of type E_ERROR was caused in line 120 of the file /home/customer/www/example.com/public_html/wp-content/plugins/event-tickets/common/vendor/vendor-prefixed/stellarwp/harbor/src/Harbor/Admin/Feature_Manager_Page.php.
It is not a perfect fix as I am unsure what caused it but my team did let me know it is working well currently
