I had the same issue. It turned out to be a security plugin obfuscating the wp_version. If you have a security plugin installed, try deactivating that first.
Since it took me 2 hours to track it down, I thought I’d share the love.
Despite the error coming up as an ACF error, it wasn’t a problem with this plugin. I’ve advised the other plugin creator about this and in the mean time turned that feature off.
