Actually per the WordPress forum rules that is frowned upon and not allowed.
Your permalinks are fine so the next most likely thing is something like mod_security or another security measure in either another plugin you have installed or that is installed on your server itself is blocking saving Custom Code.
Do this test.
In the top CUSTOM CODE TOP PHP/PHP.INI HANDLER/CACHE CODE text box copy and paste this below into that text box and let me know if the save works or it fails.
# test saving plain text
