Issue #3321620: possible fix.

modules/image_captcha/src/Form/ImageCaptchaSettingsForm.php

@@ -272,7 +272,7 @@ class ImageCaptchaSettingsForm extends ConfigFormBase {
       foreach ($fonts as $token) {
         $fonts[$token] = $available_fonts[$token];
       }
-      $problem_fonts = _image_captcha_check_fonts($fonts);
+      $problem_fonts = _image_captcha_check_fonts($fonts)[1];
       if (count($problem_fonts) > 0) {
         $form_state->setErrorByName('image_captcha_fonts', $this->t('The following fonts are not readable: %fonts.', ['%fonts' => implode(', ', $problem_fonts)]));
       }