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We recently updated to the latest version and have fatal error log entries for every order created while the plugin was active. I had a chance to investigate this today.
2023-11-22T20:50:46+00:00 CRITICAL Uncaught TypeError: flock(): Argument #1 ($stream) must be of type resource, bool given in /var/www/abc/wp-content/plugins/print-invoices-packing-slip-labels-for-woocommerce/public/modules/invoice/invoice.php:1036
Line #1035 attempts to open a file for write in the includes folder of the plugin. Since the folder isn’t writeable by the web server, the fopen returns false.
I’d suggest moving this file to the server tmp folder which is always going to be writeable.