What you added didn’t really change anything (268435456 == 256M) so it’s strange that this would clear up the problem.
There can be memory issues with ACF, but they are rare. I’ve found that most of the time these memory issues are caused by something else, many times it is an infinite loop. This can be caused by some actions and filters in WP.
I’ve found one other instance where this happens. It has to do with being logged in as an editor. What type of user are you logged in as when the error happens?
What other plugins are you running? Can you deactivate other plugins and get the same error?
