Hi heinzschmolke,
I have tested again with Twenty Seventeen (also with 2018, 2019, 2020 themes). They are totally fine with LearnPress.
Sorry to hear about your case but please help me check some server requirements like PHP 5.6 or higher. You can turn on the WP_DEBUG or easily check the ConsoleLog in the browser to find out exactly the reason why.
If you need my assistance for checking your site, just let me know, I can help 😉