I was curious because you said that acf/save_post is not working. ACF4 uses acf_save_post, but if you’re using ACF5 then there isn’t any reason it shouldn’t be working.
The code used when using acf/save_post should be different. For one thing acf passes your action function the post ID so you don’t need to go through the trouble of figuring out what it is.
When using acf/save_post or save_post hooks, there is a change of creating an infinite loop and you need to remove the action and add it again. This is probably the cause of your internal server error. https://codex.wordpress.org/Plugin_API/Action_Reference/save_post#Avoiding_infinite_loops
