This was a helpful hint, I didn’t know this, thanks.
I have now tried another test with silent mode and also enabled Arachne mode - with the same result as always. I got a bunch of strings in the inner of the thread.
I have the feeling that the problem lies in the thread itself, it seems to be too fine to be printed. I have not found any printer setting which had an influence on those strings.
The thread is of type M12x0.5. I have heared that due to its small slope, 3D printing of this thread is not easy. If someone managed to print a similar thread, please report it, I am curious to learn how to do it. Until then, I see the thread as ‘unprintable’.
Thanks for all the help.