Hi Jon,
Let’s try debugging things.
to start, can you try disabling ALL plugins except GP Premium and GenerateBlocks and see if it still doesn’t save? This is to check if the cause is a plugin conflict.
If you have a child theme, can you disable it as well? Also, if you’ve added a PHP snippet on the parent theme’s functions.php, can you remove it? To check if the cause is a PHP snippet.
Can you also try to clear ALL cache(from plugin, from server cpanel and from browser)? To be sure it’s not a simple caching problem.
Let us know how it goes.