Changing icon setting won't save

When I try to change either of the icon settings, the plugin tells me that the settings have changed, but they have not changed. The screen reloads and the icon fields are blank: indicating that the default icons are still being used.

I have been able to change and save all other fields and settings. The default icons are fine, so this is a minor issue to me.

https://ww.wp.xz.cn/plugins/slider-captcha/