Some servers use ModSecurity to block known exploits . If your editor is failing to save, your hosting provider may be blocking what it perceives as a malicious request due to outdated plugin patterns.
Nicepage 4.5.4 was released in early 2022. While no specific "named" exploit exists for this exact version, users of older versions often face risks that have been addressed in more recent updates: nicepage 4.5.4 exploit