Not working after upgrade to PHP 7.3 and Wordpress 5.5.1

After I upgraded to PHP 7.3 and Wordpress 5.5.1, the …/wp-admin/admin.php?page=jackmail_campaigns page remains completely empty, as well as Lists, Models, Stats and Parameters.
I’m using version 1.2.13 of Jackmail (says the Plugins page)
Do you see any reason why?

Hello,

Thank you for your message.
Our expert on this subject is currently absent.
He will answer you in the next few days.

Have a nice day.
Kind regards,
Solène from Jackmail.

Hello,

unfortunately I’m not able to reproduce your problem after updating your WordPress and PHP version. For this I installed an previous version of WordPress (5.2) with a PHP Version 7.0 and updated both to WordPress 5.5.1 and PHP 7.3 but had no problems after the updates.

Normally I guess that refreshing WordPress after clearing the cache and cookies in your browser could help. Otherwise you can try to rename on your FTP the plugin folder “jackmail-newsletters” temporary (to avoid to loose any data if you would deactivate the plugin in WordPress), refresh WordPress (now Jackmail is gone) and rename the folder after back to “jackmail-newsletters”. So after Refresh the Jackmail plugin should be back in WordPress and normally all pages should be shown correctly…

Best regards,
Hendrik from Jackmail