I think if you really need this exact plugin, you have to switch to LAMP stack and then configure cron as however it works with this plugin. Another idea is to maybe make it possible to disable Cloudron's cron and use WP_CRON but this is not really an option for production sites.

Then I ran the cron:

This time it didn't give any specific error message.

The plugin guys just thanked us for the feedback. I still don't have any concrete answer from them.

I'm saying this, because I've repositioned the variables in constants.php file like you said, and it still doesn't work.

@girish Then.. what should I do?.

Sorry, I wasn't clear. Please send the plugin authors the link to my comment. There is nothing we can do, they have to fix the plugin. They can easily reproduce this by running cron tasks using the WP CLI tool outside Cloudron. WP CLI is pretty much part of standard WP - https://make.wordpress.org/cli/handbook/

Do I have to tell the plugin guys that they should move the globals in constants.php file or rather use superglobals?

I have no idea how to tell them this. Can we do something from our side?

With that in mind, the code in the plugin's constant.php is like this:

<?php

$OIWE_API_URL = "http://apiv1.optimizador.io:80/";
global $OIWE_API_URL ;

The above code will break when the above file is included from a "function". This is the case when we use WP CLI where entire WordPress is loaded into a function. So, for example, the rough equivalent is:

function load() {
    require_once(".../constants.php");
}

When it's like above, the global $OIWE_API_URL keyword then says, use OIWE_API_URL from global context, which is not the same as the one you defined one line above (it's a function local variable). So, the solution is to move the global keyword before setting it in constants.php. Like this:

global $OIWE_API_URL ;
$OIWE_API_URL = "http://apiv1.optimizador.io:80/";

// .. do same for other constants in the file ..

Of course, this is why globals are not used in WP plugins. You should use superglobals like $GLOBAL or define or database etc. See https://codex.wordpress.org/Global_Variables and https://onlinewebtutorblog.com/wordpress-global-variables-step-by-step-complete-guide/

Is the error you're seeing something on our side or is the side of plugin guys?

<?php

$OIWE_API_URL = "http://apiv1.optimizador.io:80/";
global $OIWE_API_URL ;
$PLUGIN_VERSION = "1.0.22";
error_log('The version is ' . $PLUGIN_VERSION);
global $PLUGIN_VERSION;
error_log('The version is now ' . $PLUGIN_VERSION);

$OIWE_TABLENAME =  "imgoptimizations";
global $OIWE_TABLENAME;

Now, if I run via PHP cli:

root@d2c84f40-ba29-47fd-87a5-48c873006e89:/app/data# php /app/data/public/wp-content/plugins/weimgoptimizer/includes/constants.php
The version is 1.0.22
The version is now 1.0.22

So, the above works as expected. But if I now run this via WP CRON:

root@d2c84f40-ba29-47fd-87a5-48c873006e89:/app/code# sudo -u www-data -i -- /app/pkg/wp --path=/app/data/public/ cron event run my_schedule_hook
The version is 1.0.22
The version is now 

The version is empty for the second one! In fact, all the global things like global $PLUGIN_VERSION; in constants.php become immediately empty . FWIW, the global usage above seems to be incorrect. In PHP AFAIK, global is to declare that you are going to use a global variable and not that a variable has to become global.

WordPress database error Table 'c801e5ebba2fb24b.wp_' doesn't exist for query 
        SELECT *
        FROM wp_
        WHERE meta_id = 6
        AND thumb_size = 'we_original_size'
        ORDER BY time DESC
      made by include('phar:///app/pkg/wp/php/boot-phar.php'), include('phar:///app/pkg/wp/vendor/wp-cli/wp-cli/php/wp-cli.php'), WP_CLI\bootstrap, WP_CLI\Bootstrap\LaunchRunner->process, WP_CLI\Runner->start, WP_CLI\Runner->run_command_and_exit, WP_CLI\Runner->run_command, WP_CLI\Dispatcher\Subcommand->invoke, call_user_func, WP_CLI\Dispatcher\CommandFactory::WP_CLI\Dispatcher\{closure}, call_user_func, Cron_Event_Command->run, Cron_Event_Command::run_event, do_action_ref_array('my_schedule_hook'), WP_Hook->do_action, WP_Hook->apply_filters, oiwe_my_schedule_function, oiwe_optimize_concurrent_image_by_att_id, oiwe_isOptimized

The relevant code is:

function oiwe_get_optimization_data_by_attID($id){

        global $wpdb;
        global $OIWE_TABLENAME;

        $table_name = $wpdb->prefix . $OIWE_TABLENAME ;

         $querystr = "
            SELECT *
            FROM $table_name
            WHERE meta_id = $id
        AND thumb_size = 'we_original_size'
                GROUP BY uuid
            ORDER BY time DESC
         ";

        $optimizations = $wpdb->get_results($querystr, OBJECT);

        return $optimizations ;
}

In the above, OIWE_TABLENAME is null . In fact, all the globals in that file are "null" which is why we later get the error that string is expected. My PHP skillz are being put to test...

root@d2c84f40-ba29-47fd-87a5-48c873006e89:/app/code# sudo -u www-data -i -- /app/pkg/wp --skip-themes --path=/app/data/public/ cron event run my_schedule_hook

I am getting the same exception you posted.

Right now I edit the links of the screenshots and upload the images directly.

Running it like this did not make the plugin work.

The plugin cron task is this:

When the cron executes this task it gives me this error:

Is there any other way to execute that task? I have to get that plugin working. The plugin people don't know what's going on because our cloudron server is something special