`imagerotate()` php function fails to return an image object, at least with PHPv7.4 on my hosting.
More specifically, the error occurs specifying 180 degrees as the parameter value (so half of the times the captcha will not be displayed, as designed in function `generate_image()`).
I tested the relevant code form /modules/captcha/ myself and on servers I have access to (PHP in versions 7.2.24 and 7.4.32). The rotation of the background images works flawless. So this is in itself not an issue with the PHP version 7.4.
What happens on your server, what is visible to the visitor in the browser? Do you see no image or maybe only the light grey placeholder image without any text? Do you find the background images in
modules/captcha/backgrounds and a TTF-font in
modules/captcha/fonts (at least
Tahoma.ttf should be present)?
Trenne niemals Müll, denn er hat nur eine Silbe!