Notice: Undefined variable: za_button_counter

I’m getting this error msg on the latest zip-attachment version:
Notice: Undefined variable: za_button_counter in /var/www/html/wp-content/plugins/zip-attachments/zip-attachments.php on line 124

However, I’m only getting this error on one of our servers and not the on the other one. Both have identical configurations. I’n not sure why that is the case.

I have tried deleting and re-installing the plugin.

https://wordpress.org/plugins/zip-attachments/