I got it.
It is the “WPML Multilingual CMS”
1) create a new field
2) edit post and enter value in new field
3) save/publish post
-> only this field is not saved
Workaround:
I only deactivate and reactived the “WPML Multilingual CMS” plugin and everything works fine.
Wordpress: 4.6
WPML Multilingual CMS: Version 3.5.1.1
Advanced Custom Fields PRO Version 5.4.2