Thanks for your quick answer. The “fonts” folder is there. And the font “times_new_yorker.ttf” is also there. In the options I only set that one font and as already mentioned, the code works about 9 out of 10 times – so I dont think it is an issue with the fonts-folder or font-file. The error would persist.
using only one background did not solve the problem. But I figured out a solution:
I set max_font_size from 32 to 25 and the captcha now always works fine. Perhaps some letters are generated outside of the captcha with a font-size too high.
For the moment I will use this as a solution. Time will show if it is secure enough or if I have to rely on the Google Recaptcha thing. If you have experience about this, it would be glad when you share them.