I have just updated ACF to 5.2 and WPML to 3.1.9. There seems to be a compatibility issue between the two plugins. Pages and Custom Post Types flagged as translatable in WPML and containing fields created in ACF have stopped loading.
Deactivating ACF and keeping WPML active resolves the issue (but obviously, custom fields are gone)
Deactivating WPML and keeping ACF active resolves the issue (but obviously, translations are gone)
Keeping both ACF and WPML active, but disabling translations for said content type in WPML also resolves the issue
Only pages with both custom fields created in ACF and flagged as translatable in WPML stopped loading, everything else seems to be working fine.
No idea if the issue comes from ACF or WPML though.