do not access the plugin folder from external please

Glad to help, but I do not understand what this means:

“is it possible to stay inside WP?”

Also for this:

“on nginx servers the directives protect the /wp-content/ folder and your plugin is simply not working.”

I just tested on default WP and the plugin is working normally with no errors or issues.

“Maybe also blocked on apache when using security plugins or cache plugins which optimize htacccess like wpro cket”

Just tested and the plugin is fine. If you can provide steps to replicate any issue *on default WordPress install*, I will be able to investigate asap.

1) That is what some security/wprocket cache plugins adding:
# Deny direct access to .php files in the /wp-content/ directory (including sub-folders).
# Note this can break some poorly coded plugins/themes, replace the plugin or remove this block if it causes trouble
location ~* ^/{FOLDER}wp-content/.*\.php$ {
deny all;
}

2) Stay inside WP means to not call the folder/file like https://dummy.santegra-international.com/wp-content/plugins/simple-ajax-chat/resources/sac.php?ver=20220720

Thanks for clarifying, I will add it to the to-do list for a possible future update. I appreciate the feedback, @tobias_conrad. Feel free to post again with any other ideas, questions, etc. Always glad to help.