Various PHP warnings and errors even in test mode

Hi everyone,
I need urgent help!

Working on a website based on the commercial theme Enfold template.

Whole site runs very well, no error logs.
Now I need an event ticket system, so I chose ‘The Events Calendar’ (https://ww.wp.xz.cn/plugins/the-events-calendar/) with ‘Event Tickets’ (https://ww.wp.xz.cn/plugins/event-tickets/) and ‘PDF Tickets’ (https://theeventscalendar.com/extensions/pdf-tickets/) for free testing, before buying the paid Calendar & Tickets plugin.

Installed the plugins and 1st thing was, that I always get the following PHP error when opening ‘Events/Options (or Settings in EN version?):

Warning: sprintf(): Too few arguments in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/the-events-calendar/src/admin-views/tribe-options-display.php on line 53

Besides that, it generally seems to work so far…

Then I activated the PDF plugin and so far this works: PDF is being generated and attached correctly. BUTwhen I view my RSVPs I can click on ‘PDF tickets’ to view them by myself. THEN I 1st got:

Fatal error: Class 'Tribe__Tickets_Plus__Commerce__WooCommerce__Main' not found in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-pdf-tickets/index.php on line 342

Did exactly all the https://theeventscalendar.com/knowledgebase/testing-for-conflicts/ steps and then got:

Fatal error: Uncaught Error: Class 'Tribe__Tickets_Plus__Commerce__WooCommerce__Main' not found in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-pdf-tickets-1/index.php:342 Stack trace: #0 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-pdf-tickets-1/index.php(863): Tribe__Extension__PDF_Tickets->get_attendee_id_from_unique_id('5a6a671a5e52364...') #1 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/class-wp-hook.php(286): Tribe__Extension__PDF_Tickets->load_pdf('') #2 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #3 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/template-loader.php(12): do_action('template_redire...') #5 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-blog-header.php(19): require_once('/is/htdocs/wp11...') #6 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/index.php(17): require('/is/htdocs/wp11.. in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-pdf-tickets-1/index.php on line 342

It was worst when I tested the plugin ‘Advanced Options for Adding Events to the Main Blog Loop’ (https://theeventscalendar.com/extensions/advanced-options-for-adding-events-to-the-main-blog-loop/):

Fatal error: Uncaught Error: Class 'Tribe__Settings_Helper' not found in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-main-blog-loop/index.php:62 Stack trace: #0 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/class-wp-hook.php(286): Tribe__Extension__Main_Blog_Loop->add_settings('') #1 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #2 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #3 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-admin/admin.php(156): do_action('admin_init') #4 /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-admin/plugins.php(10): require_once('/is/htdocs/wp11...') #5 {main} thrown in /is/htdocs/wp1142725_C5XG7JLS1K/www/ah26/wp-content/plugins/tribe-ext-main-blog-loop/index.php on line 62

^^backend dead, only this error message. Had to deactivate the plugin manually to return backand access again.

What’s wrong here??

WP 4.9.2, all above Plugins latest versions (just downloaded/installed from original source), WP in test mode with all other plugins disabled and theme twenty sixteen on – all as required in https://theeventscalendar.com/knowledgebase/testing-for-conflicts/

Please help me! Any suggestions or hints??
Have to get the website running very soon.