Thumbnails don't appear

Two more diagnostic steps:

1. Make a test catalog and import some of the same images into it. Does it happen with the new catalog?

2. Please do the LR menu command Help > System Info, click the Copy button in the upper right corner, and paste into your reply here, so we can see exactly which versions of hardware and software LR thinks you're running and important LR options that are set.

I already adjusted the profile to sRGB and deleted the preview coaches. But it´s still not working

[View this post in your web browser. It contains formatting and images that don't appear in email.]

I've already adjusted the color profile

You don’t specify what you did. But try this diagnostic step with the display profile:

http://www.lightroomqueen.com/articles-page/how-do-i-change-my-monitor-profile-to-check-whether-its-corrupted/

If that doesn’t help, try deleting the catalog’s preview cache:

https://helpx.adobe.com/lightroom-classic/desktop/kb/lightroom-gives-error-preview-cache.html