Illegal string offset

Hello,

I need help as I am currently having an issue with your plugin on a client’s site.
Indeed, as you can see on the address provided, the plugin does not work at all and the form to leave a message is displayed below the footer and without CSS.
If WP_DEBUG is enabled, I see errors like:

• Warning: Illegal string offset ‘theme’ in /wp-content/plugins/wpdiscuz/forms/wpdFormAttr/Form.php on line 138
• Warning: Illegal string offset ‘theme’ in /wp-content/plugins/wpdiscuz/forms/wpdFormAttr/Form.php on line 160

Could it be an incompatibility with the theme (Hub: https://themeforest.net/item/hub-responsive-multipurpose-wordpress-theme/31569152)?
What’s weird is that on the developing site, which has the same theme with the same plugins and the same versions, it works perfectly.

Can you help me ?

Thanks in advance.

The page I need help with: [log in to see the link]